Shallan's Memory ability to capture an image doesn't seem to be working exactly like an eidetic memory...
...because she can erase it by drawing. Is that tied directly to the magic of Lightweaving? And if so would other creative Lightweavers have similar abilities.
If you look at the epigraphs there is a big hint on this, where it talks about it and yes... I have problems with eidetic memory, just because scientifically most scientists say this is not a real thing. So I would say that whenever-- Because of my knowledge of the science of it understand that you would need some sort of magical enhancement to be able to do what she does.