Yes, Jay makes a good point. There are two limitations apparently, no larger in display dimensions than 150x150 pixels and no bigger than 350kB in file size. As Jay suggests, a jpg and gif of 150x150 or less will always fit the file size parameter, but maybe not with BMP.
PS, just ran a quick experiment and it seems that even in bmp format the 150x150 limitation may be all you need to worry about.