I used to do that too but Unsplash has prettier pictures and creates a more satisfying aesthetic style for me. But Pixabay has flexible pictures for every topic! So, before I used to grab all my pictures from Pixabay, but now I only look there if I have a specific need that a stock photo wouldn't cover.

I'll see if Pixabay has a similar API that can be triggered with a bookmarklet and if it does I'll make a tool for that as well.

NinjaEdit: Just remembered that Pixabay doesn't allow hotlinking unlike Unsplash, so a bookmarklet would have to resize, then upload to imgur and that wouldn't be optimal.