My opinion is that you shouldn't buy a Rickman autograph on a Potter image, unless you have $3000+, because that's the only way you will get one that is both real and uninscribed.
Prior to his passing an uninscribed Prof. Snape photo from Harry Potter sold for $1600. In Rickman's own words he would sign Potter memorabilia when the hype died down.
Unfortunately, he passed before the hype did. There are enough die-hard Potter fans out there that want to finish their collections, and just not enough signed early material to do so. I actually turned down a $3000 offer.