Why do some backgrounds work and otherschool don't?

A

AC Question

I have a Galaxy S6 from Verizon and I use a Steampunk theme from +Home Launcher. There are certain pictures that I can use as a background for the home/lock screen and then there are others that I can't use. I get the message "Unable to set image as wallpaper. Image deleted."

Can anyone explain why I get this message on certain pictures and not on others? Thanks in advance!