[@The Whacko] [url=https://www.artstation.com/]Artstation[/url] tends to be a good place for various kinds of artwork. 

[url=www.zerochan.net]Zerochan[/url], [url=http://safebooru.org/]Safebooru[/url] and [url=https://www.pixiv.net/]Pixiv[/url] (if you can read the moonrunes) tend to be good for more [s]weeb[/s] anime/manga-oriented artwork.

I personally don't use them often myself, but [url=www.deviantart.com/]DeviantArt[/url] and [url=https://www.tumblr.com/]Tumblr[/url] are likely good spots to find images of superhero OCs, too. I don't really "get" the interface for Tumblr though.

[@Er0r] I recently updated the OOC with some examples of the art style I'm looking for for this RP. [@Dblade26]'s character is also an example of fitting character art.

EDIT: Now that I think about it, [url=https://uk.pinterest.com/]Pinterest[/url] might be a good place to search, too.