Building a sprite sheet.

I have my animation stored in 14 png files.
Does somebody know a software or something to store them in a single spritesheet?

IrfanView :

http://www.irfanview.com/

And, just FYI, the opposite: http://www.alferdspritesheetunpacker.forkandbeard.co.uk/forkandBeard/apps/AlferdSpritesheetUnpacker/Download.aspx