Let me ask you a couple of questions.
Have you ever just tried asking your mother directly? Or saying something similar to "why do I look so much like my uncle?"?
Also, some would say it stands to reason that you would look like your uncle, after all you are his sisters son, there's bound to be a resemblance, especially with you both being male.
You sound like you're not sure if your stepfather is your father or not, you're just going by the fact that he said he's not your father. Why don't you start there? Get his dna and see if he is your father. If it comes back that he is your father, then you can stop everything knowing the truth, however if you find out for sure that he is not your real father, you can just dismiss him and all of his abuse, and you can tell your mother that you know 100 percent that your step dad is not your father, and you want to know who your father is. You have a right to know who your father is. Period.
As far as being able to tell if that is your real father through pictures alone would be impossible. Forensic science has facial recognition programs, but they couldn't guarantee them because think about how many children don't look like either of their parents? The only way to be absolutely sure is DNA.
I welcome your thoughts