There are any number of questions asked where the description, or the formula, is still totally unclear without a diagram. For instance what's the impedance of an RLC circuit? Well OK, what configuration?
It may be appropriate to use the existing '5 votes to hold' process, but perhaps have an additional radio button there specifically for 'lacking a schematic diagram'. As the case is more clear cut, maybe fewer votes could necessary for that reason? Perhaps when held for that reason, the 'add a diagram' button could be highlighted in some way?
Could it possible for the system to automatically release a question when a diagram is added? This would avoid exacerbating the on hold release review queue situation. Or is that non-trivial server side code that's more trouble than it's worth? The system would not have to judge the quality of a diagram, I'm assuming that a totally inadequate one would be re-held by humans.