Can 'Fullscreen' and 'Full Screen' be used interchangeably? As in
Funny pictures in fullscreen
Funny pictures in full screen
The context is a website, with full screen images in a fullscreen website.
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."
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."
These are terms not well defined in any reliable reference source that I know of. One suitable definition comes from the Computer Hope site.
Fullscreen is a display format that changes the original display aspect ratio to fill the entire display area of the monitor/tv. This may be done through clipping of edges, or stretching the image to fit the display, and therefore changes the final appearance of the image or video.
Full screen indicates that the application or image occupies the entire screen, with all other applications in the background. This is in contrast to windowed applications, which only occupy part of the display area, and allow access to other applications on the desktop. It is also commonly written as full-screen.
Images can be fullscreen and/or full screen. An image shown in full screen that does not have the correct aspect ratio as the monitor may have thick black borders around one or more edges. The same image shown in fullscreen will appear distorted, as it may be stretched or shrunk in one dimension to fit the screen, or it may have one or more edges cropped.
To make it more confusing, and image can be fullscreen within a windowed application. Fullscreen formatting used to be fairly widespread (no pun intended) in TV/video applications, but is increasingly less common these days, as there so many different monitor/display sizes and the result is usually an image or video that is distorted or cropped.