To make this clear: All options are possibly correct depending on context. However, in this context, #3 is most appropriate.
#1 would be correct if this sentence were not in the infinitive like.
"Look! This is a picture of you as a baby.
#2 would be correct if you had sent someone a picture of your car and wanted to see that person's car
"I've send you a picture of my Dodge Viper. Can you send me a picture of yours?"
#3 is the the correct answer here because you have the imperative (which) omits the pronoun but implies it. If you change the imperative to a modal you would get this:
"You should send a picture of yourself."
The rule is that if you already have pronoun then the next pronoun (referring to the same person/thing) needs to be reflexive. Examples:
"The dog bit itself."
"You see yourself in the mirror."
In the same sense you get:
"He refused to send me a picture of himself even though I sent him one of myself."
"One should think twice before uploading pictures of oneself."
When you have the imperative the pronoun is simply implied. Example of uses of the imperative in this way:
Write a short description of yourself.
Upload a picture of yourself.