I think a screencap is when 你 just take whatever is on the screen and make it into a picture. I mean like for a screencap you'll literally take your camera and snap a pic of the screen, 或者 you'll hit ALT+PrtScn to make a 帽 of your computer screen. Plus, screencaps are of the entire screen, normally. No cropping 或者 whatever.
I think 照片 are 更多 edited and planned, rather than a 随意 pic of a 随意 scene.