You are correct that Silent Image can create a wall. Both spells belong to the Illusion school, with the "figment" subtype, and allow a will save to disbelieve the illusion. But each spell has its advantages and limitations.
Silent Image is a temporary, cheap, and flexible illusion.
Consumes a level 1 spell slot.
On the bard, magus, and wizard/sorcerer spell lists.
Range is Long (400 ft. + 40 ft./level).
The maximum size scales with the caster's caster level ("four 10-ft. cubes + one 10-ft. cube/level").
Requires concentration to maintain. A caster could have only one Silent Image active at a time.
If someone succeeds the will save to disbelieve the image, then they can partially see through it. According to the rules on illusion spells, it will appear translucent upon a successful will save, because the Silent Image is a figment.
A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.
Illusory Wall is a permanent and effective illusion of a wall or similar surface.
Consumes a level 4 spell slot.
On the wizard/sorcerer spell list only.
Range is Close (25 ft. + 5 ft./2 levels).
Illusion size is constant (1 ft. by 10 ft. by 10 ft).
It has permanent duration, where as a Silent Image requires concentration to maintain. With enough castings of Illusory Wall, you could eventually create a permanent illusory castle!
Unlike other figments, even if a creature succeeds the will save, the illusion is still effective. Someone may know the wall is an illusion, but they still cannot see what's on the other side!
Although the caster can see through his illusory wall, other creatures cannot, even if they succeed at their Will save (but they do learn that it is not real).
Images created by silent image have no AC RAW
The result of silent image is not a creature or an object; it is an image.
You create the image of an object, a creature, or some other visible phenomenon that is no larger than a 15-foot cube. The image appears at a spot within range and lasts for the duration. The image is purely visual; it isn't accompanied by sound, smell, or other sensory effects.
Nowhere in the spell does it describe any rules for assigning the image an AC.
Compare this with mirror image which says:
A duplicate's AC equals 10 + your Dexterity modifier.
This is a special case because the illusion in mirror image is moving with the caster and thus makes sense to have an AC based on the caster's stats. However, this doesn't make much sense, for example, for an illusory stationary potted plant to use that same rule. So this is not going to work as a general rule for assigning an AC to illusions. However it does show us that if a spell intended to assign an AC to an illusion it would say so in the spell.
Since silent image doesn't describe any kind of AC and there is no general rule for assigning AC to illusions, then we must conclude that the things created silent image simply have no AC.
It also makes sense since AC is:
how well your character avoids being wounded in battle. Things that contribute to your AC include the armor you wear, the shield you carry, and your Dexterity modifier.
And an image obviously has no effective armor, can't dodge attacks, and has no dexterity. So it makes sense to have the enemy hit it without needing to roll against an AC.
The same holds true with minor illusion and major image as well.
So how does this work practically?
If an enemy tries to attack a silent image, most likely the DM will simply say that they interact with the image and thus know it is an illusion with no roll needed. This would seem to be the option that the rules would suggest.
Even though deducing an illusion using intelligence requires a check this still makes sense. Physically interacting with an illusion is by far the easiest and most straight forward way to check if something is an illusion. A good illusion is very difficult to discern by sight, but very few will stand up to any sort of physical interaction at all. Also, the spell says nothing about having to make any sort of check to interact physically; if the DM allows it, it simply happens.
DMs can assign the images AC in cases where it makes sense
Obviously, illusions have a huge variety of uses and use cases. Even though the rules imply the general rule is that images have no AC, the DM may encounter cases in which it does not make sense for an attack to automatically succeed.
In cases like these if the DM wants to simulate a miss chance or some other possibility that would make AC useful they may just have to assign the creature an AC and have the monster roll an attack against that. Then, if they hit, reveal that it is an illusion due to physical interaction. However, they will have no guidance from the rules about what AC to assign. They'll have to adjudicate it on a case by case basis.
Best Answer
A face can't be changed with silent image, but other options exist
Because the spell silent image is in the figment subschool of school of illusion, the spell "cannot make something seem to be something else." An illusion spell of glamer subschool like the spell disguise self, for example, can make a drow's face appear to be a different face, as spells of that subschool can make things "look, feel, taste, smell, or sound like something else."
However, the Disguise skill is usable untrained, and the drow's friends—if he has any—can aid him on the check… or, if a friend is more capable than the drow, the friend can make the Disguise skill check on the drow's behalf, disguising the drow. With enough friends—and the GM's permission—the drow should be able to roam the town unhindered until some high-level watchman with a super-high Perception modifier penetrates the disguise anyway. A disguise created by the Disguise skill has the added advantage of not being magical, so the spell detect magic won't reveal it, the spell dispel magic won't collapse it, and the spell true seeing won't penetrate it.
Also, as a level 5 PC—if this avenue is really important—, the drow could invest 1,800 gp of his 10,500 gp PC wealth in a hat of disguise. Just make sure the command word is utterly mundane so the drow doesn't look like a weirdo when he must reactivate the hat every 10 min.
Finally, confirm with the GM that in the GM's campaign that drow are still feared and hated. In many settings, drow are just different elves, and folks won't, like, kill them on sight or anything.