It shouldn't affect database performance, because the speed of a database is based on indexes. But there is one gotcha that you might want to look out for: there's that file size limit (2G?) that might be violated if you had a lot of really big pictures.
In my apps, I've only had one picture per page, and it makes good sense to put it in the database. But, there's more than one way to do it...
Using the database means you don't have to worry about naming the images or overwriting images or making sure the file system is set up right.
But it will increase the load when backing up-- but then again, would you be backing the images up if they are in the file system and not on the database? There may not be an appreciable difference. BTW, mysql backups can be accomplished very quickly using mysqlhotcopy. (moving the files to another place can take some time, though...)
