Sort of a mixed bag here . . .
I tend to think of ogres as "small" giants - perhaps 2-3 times the height of an average man, stout (for their size), muscular, somewhat neanderthalish or homo habilis looking in appearance, rather primitive in mental capacity, and so forth.
Orcs are "small" ogres (ie: half sized orgres, large-ish man sized), but I also tend to think of them as having short tusks or tusk-like teeth - especially in their lower jaw (rarely their upper jaw). I think my view of them was affected in part by my imagination as I read various old stories, myths, and fairy tales, and in part from movies (esp the tusks part).
Trolls are right out of norse myths and tolkien - hairy tall humanoids with rock hard hide that become stone if they are exposed to sun light.
Medusas - I used to think of them as found in clash of the titans, then my view altered as I became interested in mythology in my early adolescence. I literally read dozens of books on mythology during that period, and I recall reading some part that suggested that Medusa's curse was not an alteration of form (other than her serpentine hair), but rather that all that viewed her directly became stone. So, unlike the images in the MMs, I tend to think of Medusa as a woman with serpents instead of hair on the top of her head. She might be normal in appearance, but considering that at least one deity was attracted to her, I tend to think of Medusa as being beautiful - but in a way cursing her own beauty for the curse that it brought upon her.