Learn English – Why is “herself” required in this particular sentence

Why is a reflexive pronoun, i.e. herself, grammatically required in the following sentence?

• I gave Susie a picture of herself.

Compare with:

• I gave Susie a picture of her.

This sentence doesn't seem to be able to mean I gave Susie a picture of Susie. It means I gave her a picture of someone else.

I've looked at the following page, but this type of usage doesn't seem to be covered there:

• When is it correct to use “yourself” and “myself” (versus “you” and “me”)?

The answers there suggest that we use reflexive pronouns when the subject and object of the verb are the same entity. Herself isn't an object in the example above, and Susie is not a subject either. The syntax in this example seems to be entirely different.

Also, why is the following sentence not grammatical?

• I went there by me.

… as opposed to:

• I went there by myself.

This last example, of course, is perfectly fine. So the question here is why does this so-called 'idiomatic usage' contain a reflexive instead of a normal pronoun.

Lastly, why is a reflexive required in this imperative?

• Do it yourself.

Compare with:

• Do it you.

Note that yourself isn't an object in that first sentence. (It would be if the sentence was Watch yourself, for example. The object in the example, however, is it.)

What then are the grammatical rules that stipulate the use of reflexive pronouns in these examples?

Edit note

I've changed the order of the examples here to take the focus off the probable emphatic usage in the imperative (although I'm still interested in the grammar here). I've also added a third example, which highlights more clearly the two issues I am most interested in hearing about:

1. Examples where we need a reflexive pronoun, but the pronoun isn't the object of a verb.
2. Examples, where the antecedent, the original person or thing that the reflexive is duplicating, is not the subject of a verb.

Any answers with relevant observations and or research would be very gratefully received, whether they address the whole question, or just parts of it.

Observations about the imperative example

Although I'm most concerned with the photograph example, I hereby offer some observations about the imperative one. Some of the interesting comments below suggest that yourself here is an emphatic version of you. As RegEdit's answer points out, the example with yourself seems to be contrastive. The sentence with you, on the other hand, does not give the same reading. I wonder why that is?

I note in passing that yourself seems to be an adjunct ('adverbial') here. This makes it different from another emphatic use of pronouns, which is when a pronoun appears as the subject of an imperative:

• You do it!

I originally gave the imperative example for two reasons. Firstly, the reflexive pronoun here is obviously not the object of the verb. Secondly, however, I gave it because there is no antecedent word in this sentence which could be the subject of the verb, or, which represents the same person as yourself. I am curious as to whether the rules requiring reflexive pronouns in emphatic usages, are basically the same ones that require us to use them the rest of the time. In other words, I return to my original question: Why is a reflexive pronoun required in this case?