We currently have ascii-art and graphical-output tags. There doesn't seem to be any tag which is unambiguously for challenges which involve output in an image file format, so they sometimes get put in graphical-output and sometimes (e.g.) don't get any tag. Moreover, some challenges which ask for ASCII-art output (e.g. 1, 2) are tagged as graphical-output but not ascii-art.
Inconsistent usage is obviously a small problem, which can be fixed by agreeing a standard and then editing tags as appropriate. But I think it's also a problem that tag completion from "image" doesn't send you to "graphical-output".
I am tempted to create an image tag and retag everything in graphical-output as either ascii-art or image, or both where the question gives the implementer a choice between the two (e.g.). However, the name doesn't strike me as adequate, because there have been questions which take images as input but don't produce them as output.
Maybe it's sufficient to make image a synonym for graphical-output for the time being and to change that later if there's much call for it to cover graphical input too.
Suggestions?