I did a search on "animal rescue public domain images" and got some interesting images that might possibly be used for a cover (+ text of course) but you'd need to be able to manipulate images etc with photoshop or gimp or something. (this is the process I use for the most part)
https://www.google.com/search?q=anim...w=1222&bih=783
I thought this image in particular might fit your topic:
