Mod note: Tumblr is about to start inserting ads into blogs on mobile and on the standard theme. I have preemptively turned this option off on this and all my other blogs (here’s how to do it) not because I want Tumblr to go bankrupt but because knowing Tumblr the ads will be intrusive and annoying and make the platform impossible for some people to use.
Also, most of images I use are licensed by the museums and galleries under the condition that they not be used commercially. I can’t monetize content I don’t own the rights to! (Tumblr is supposed to be adding revenue sharing at some point). Even if monetization isn’t the primary purpose of the blog, it still seems inappropriate. I don’t think Tumblr technically should be monetizing that either, but I’m not sure how that one shakes out.