Some years back, it was very popular to offer free images for other webmasters to use, such as by hotlinking those images. Many times, this technique would be used on free graphics sites. And, for awhile it was a very popular SEO technique. But, we're talking about five years ago when this was popular. Is this technique still good to use? I have not read where using this would get your site penalized. However, a lot has changed in five years. I would feel uncomfortable relying on any shady techniques that were popular five years ago. You could try it out on one of your less important sites to see what results you get with Google and see if those results actually sustain.