Learn English – Is “without” a preposition or an adverb in “She left the hotel without paying the bill”

We normally speak of someone being in a photograph rather than "on". What may change things slightly here is that you are referring to a particular part of the photo, the left. We always say "on the left", never "in the left", as that is simply incorrect.

What you see as the left side of a photograph isn't technically the left. The person to the left in a photo was standing on the right when it was taken! So if you are describing a person's position in a photograph technically I suppose you should say "on the left of the photo", because you are referring to the printed photograph rather than what it depicts.

Your definition of an adjective is somewhat misleading. The classic definition is that an adjective MODIFIES a noun. That may mean assigning a physical characteristic such as color to a noun, e.g. "the red ball," but adjectives are not limited to physical characteristics. "Modify" means almost anything that helps narrow the meaning of a noun. So in the context of your example, which is a comparison, "attention" could conceivably be more or less or the same as it was previously. The sentence would literally be meaningless if the range of meaning of the noun "attention" was not restricted to one of those three options. Because "less" in this example affects the range of meaning of a noun, it is being used as an adjective.

EDIT: "Less" is one of a small class of modifying words in English that do not morphologically indicate when they are being used as an adverb or an adjective by the presence or absence of the suffix "ly." So the word "less" may be used as an adverb: "She is less exuberant than she was in college." In that example, "less" is restricting the meaning of an adjective rather than a noun and so is classed as an adverb rather than a noun.