I would advise to just use common sense and ask permission first - say 'please' and 'thank you'. It's free to ask! As a reminder, here is a 2007 post about my number one frequently asked question.
And if you want to read more about a recent example about the consequences, check out this post by Andy Baio: http://waxy.org/2011/06/kind_of_screwed/
In compensation, they were seeking "either statutory damages up to $150,000 for each infringement at the jury's discretion and reasonable attorneys fees or actual damages and all profits attributed to the unlicensed use of his photograph, and $25,000 for Digital Millennium Copyright Act (DMCA) violations." After seven months of legal wrangling, we reached a settlement. Last September, I paid Maisel a sum of $32,500 and I'm unable to use the artwork again.