Why screenshot is 截圖, not 屏幕 圖?
截 in 截图 (noun) means something similar to 'that is taken', so 截图 in a word means 'the screenshot that is taken'
This becomes clearer if I use 截 as a verb in this context 请(please)你(you)截(take)一张(a)图(screenshot)
The idea of intercept in this case can be explained as that a screenshot is a static image, so you are intercepting something that is running in a moment of stasis.
But 截图 is always used in the context of screenshot, it is assumed that 图 in this case already means (screen)shot. Much like how the word 'image' can change what it refers to in specific context. You can't use 截图 in the context of taking a photo.