Ever jumped into the deep end of a pool?
And discovered someone let the water out?
eeek!
You are talking about CAD software, where you can draw something, and the dimensions are part of the drawing, and can be made part of the picture of the thing you are drawing. Good software that does this well is not cheap, and you are not going to learn how to use it in an hour.
I can't even think of anything to recommend, there are some free things, but they are more targeted to animators, they are complex as hell, and they are probably overkill for what you want, tho, I don't really know exactly what you want to do. If you just want to add a few lines to a photo and some text to show size, a paint program is probably easier, at least as far as learning curve, but it is a pain to work on a picture that was, as the computer isn't helping you at all, it doesn't know anything about pictures, or how big anything is, you have to do all that yourself. The henmeral idea of CAD software is to tell the computer all about the thing you are drawing, in exchange, it helps out by being able to do nice things like adding dimension lines, etc...
