You would need to use some type of photo editing software. There are lots of free ones out there but I haven't tried too many because I have a fair bit of software like Adobe Photoshop and Macromedia Fireworks that I use for image editing.
Here is a link to a site with lots of choices and reviews. Some are freeware and some are shareware. If you can find a free one that works well for you I'd go with that.
http://www.tucows.com/imgedit95_default.html
One I have tried that is OK is called Irfanview, I think you can get it from download.com. The one I really like for resizing pics is Mihov Image resizer. Works great for shrinking lots of images. It appears to also do conversions but I have never used it for that. I do really like the product though. The website has lots of Mihov utilities and best of all they are FREE
http://www.mihov.com/eng/#ir