I feel your pain. Happened to me often - I clicked on a photo in AvB, and a second later, I thought: wait, what was that? And then it's gone...
Do you have some more info on the photo you are looking for?
Current United colors, or an older livery?
Daylight or dusk / night shot?
Were all aircraft in the picture UA 737s?
Ideology: The mistaken belief that your beliefs are neither beliefs nor mistaken.