I think the best solution to this SWR is indeed simply
Note that, of course and obviously, the OP means ordinary representational ("non artistic", let's say) drawings.
Imagine, say, a board game where you have to draw an object (cat, dog, car, house - whatever) and the other players have to guess what it is. Or, the line drawings of objects in small children's books. Or playing Scribblenauts.
I feel in that situation, you'd say things like "It's unclear what you've drawn!" "I have no idea what that drawing is!" "What the heck were you trying to draw here?"
That milieu is the most analogous in the question as actually put, "Handwriting is to illegible as drawing is to what?"
Really, "unclear" is the only word that works there, I think.
If, as many folks have commented, the OP meant Jackson Pollock paintings, the whole question is vague and should be deleted. The question ("Handwriting is to illegible as drawing is to what?") is only meaningful if you're talking about "drawings" in the sense of straightforward line drawings of things.
Unrecognisable is wrong because it usually means, that the visual process by which you are viewing some thing (perhaps a person, car, drawing, animal, house - or any thing) is of low quality such that you cannot recognise what the thing is. This is utterly unrelated to the drawing being so "bad" that it is not clear which object is being suggested (ie - "unclear").
indecipherable applies more to "reading' (see the dictionary definition; note "ciphers". if the word was more like "in-line-able" it would be suitable here.)
unintelligible means impossible to understand. it only makes sense when a meaning is attempting to be communicated. So, a sentence can be unintelligible. in contrast a single character (like "d") is not trying to communicate a message, it's a single symbol - which may be unclear