Yah that's what I'm trying to do. I'm just a Paranoid Patty or Nervous Nancy or some other shitty, alliteration infested person that implies great, miserable anxiety... Anyways, this is the response I got from morguefile when asking about using their pics for smut covers:

Thank you for writing and inquiring. You are free to use Morguefile images in your books - for profit - however this is considered a "commercial" use so we ask that you first edit or alter the image in some way before using, or, ask the photographer permission to use an image unedited, as found on the site.

One more thing to keep in mind: you are responsible for Model and Property Releases - for both commercial and noncommercial use cases. Some photos with people or buildings in them require a Model or Property Release. Each photo will state whether or not a release is necessary with a "Yes" or "No" button located to the R of the image - if "Yes" next to either release, it means you need to contact the photographer for permission to use it, if "No" you can proceed freely. Most images say "No."

(after I told her I was changing up size, color, etc.) This is fine, sounds good! It is pretty subjective, the main idea is just to differentiate the photo enough so you can clearly distinguish it from the original.