How much RAM do you have in your machine?
The point of a sleepimage is that, when running on reserve battery power, it stores what contents are in the RAM to the harddrive as to not run the battery down even more by using the RAM.
If you use a portable.. I wouldn't delete it.
If you're using a desktop that doesn't run on Battery, from all accounts I can remember, you can delete it. But.. I would have to do a little more research on that first.