firstly, just don't do it and you will have now problems as there is enough free stock out there anyway.
http://www.bluevertigo.com.ar/ is a great example of no need to cheat.
my knowledge of an image usage ( having a degree in photography ) is that if you manipulate it by 25% the copyright stops, if you use 25% of the image then copyright stops, but i am not a lawyer so if someone complains then i strongly suggest you remove the image immediately.
in the blogs we do we use other peoples images and then give them a link credit, that seems to help as we have had no complaints.
also use finding an image to your advantage, contact the person and ask their permission stating you will give them a link for using it, you get a GREAT image for your article and they get a link, you may find they then want to give you more images for more articles THUS giving you more ideas for CONTENT to boost your site with google.
tim