What if it's something he remembers--but all he remembers is that image? He can't remember even the physical sensations or emotions, not even what it is that's smeared all over him, and he's not good enough at articulating or verbalizing yet to really describe it to a therapist or anything. Finally he tells Steve that there's this thing he remembers and it seems important but it's just an image and could Steve try to draw it, and maybe Bucky can guide him to the way it looks in his head, so Steve can help him figure out what it means.

Cue Bucky working with Steve like a sketch artist, correcting Steve's drawing, and Steve gradually watching this scene take shape under his hands, Bucky saying "no, face more like this--" and Steve being the one who realizes that Bucky is describing an expression of pain, or fear, or total desolation, Bucky saying "something wet, right there--not water, but kinda clear--" and Steve realizing it's come, realizing that he's drawing Bucky in the aftermath of a brutal rape and Bucky doesn't know. Maybe he still doesn't understand after he's seen the finished image, and Steve has to decide whether to explain it to him or just let him shrug and walk away, having gotten the image out of his head.