It depends on certain things. First off, you would need either hacking knowledge or a compatible graphic/tile editor (one such example is Tile Layer Pro). Secondly, assuming you're going for the latter, you also need to see what the size of the graphic is. For example, in an RPG such as Pokemon, the overworld sprites are about 16x16 pixels in size; this means that the sprite can only be 16x16 pixels big. On the other hand, you could probably change the size of the graphic if you have hacking knowledge, but that would certainly be a huge endeavor (not to mention that you'd have to edit other bits of programming to compensate the changes).
In short: It's possible, but doing it may end up being more trouble than it's worth.