Why am I so pathetic.......?

It is possible to still be aroused by somebody even if you had a big fight and could never speak again. When looking at a picture, you see a snapshot of a smiling face (in most cases). It triggers memories of the good times. You saw her, thought of your old good times together (subconsciously or not) and then were turned on by them. It's not unusual. If I were you, I would just avoid looking at her pictures. Since you don't want to start things back up with her, it's just best to avoid her, especially if she is causing you to have contradicting feelings. Good luck!