Yo, I just wanna say that I think It's better to have a more FUller image of your OC on the thumbnail and not just their face. I can't tell if I want to draw an OC from just their face, and often skip right over them. I don't even decide "I'm not interested in this OC" I'm going "I can't even tell what this is so I'm going to skip".

I suggest having a thumbnail that includes as much of the main body as possible of the character so that people can clearly see if they have an interest in the character and its features. A character is more than just a face, and for people searching to find cool OCs to draw, the more clear visual information the better. You might know what your OC looks like, but 99% of other people playing probably don't, and folks tend to not like taking the time to click further in this Onslaught of OCs to find out if they do have an interest in them.

Generally I'd say Having about 2/3 of the character in the thumbnail is good enough for other people to make a clear dicision about interest right away.