go to file, save as, click on the drop down box and select PNG.
when this pops up just hit ok x.x
-----
And there ya have it, a nice simple mikuru siggy =D
I hope you learned something or got some ideas from this ^_^
Not the best tutorial... if you've never even opened Photoshop this isn't very good... this is more for people who know how some of the tools work but are still clueless.