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I am sure if you searched for " ~images -images" before Google decided to drop the ~ operator , pictures would have come up as an alternate keyword and so, they would have seen it as a close match for pictures and served your site for your query. It was one of my fav tool for finding alternate keywords to optimise for , sadly that is gone now As for on page vs off page both are important .. it not an "either or" condition you should do both to get good results , especially if the competition is strong.

Don't forget about Pinterest as that is all about pictures and funny pictures are actually the most shared pictures on Pinterest. I'm sure that people will pick up on your website if your pictures get shared often enough.