There are hundreds of legal programs around for image manipulation, and even more illegal ones (by that I mean "cracked" professional stuff).
I use PaintShop Pro. If you want to change the format of a file, you simply Save As, give it a name, then change the file type in the drop down list. It's also helpful for resizing files.
Try
www.tucows.com, there is a lot of free stuff there. Specifically for image stuff try
http://www.tucows.com/grap95_default.html. Or search in Downloads on
www.zdnet.com.
Beware, some products are Try and Buy, and may have limited functionality (eg can't save) or time expiry, until you pay the (sometimes paltry) licence fee.