It's easier to think of a single picture as being equivilent of a single, though extensive, chapter. Of course, everyone is different, but for me, a chapter with an illustration takes the same amount of effort as two chapters. I think illustrations really add to a chapter, as it can show things that are impossible, or are really difficult or vague, to explain. So I think you should go for it, try it at least once; I'd definitely check it out if you did.
And uh, if you need someone to make a few illustrations... *wink wink wink*