Fullscreen or full screen
Solution 1:
Fullscreen is a mode something is in. "Put it in fullscreen"
Full screen is describing when something should take up the full screen. "Make it so that its in the full screen"
From your example, "Funny pictures in fullscreen."
Solution 2:
In the adjectival form, fullscreen = full screen so "Funny pictures in fullscreen (mode)" is the same as "Funny pictures in full screen (mode)."
However, there is also a school of thought that advocates that the adjective form is fullscreen (or full-screen), not "full screen."
see:
Wiktionary
YourDictionary
PCMag