You can in Adobe Photoshop!The version I am using for this guide is Adobe Photoshop CS2, but it will probably work for all photoshop versions.
First, open up your image in adobe photoshop, it's best if it's saved in PNG in it's best quality.
Once you've done that, click file ) Save for Web...
Once in that, change the options to the same as below, then click save.
After that, upload it onto an image host, I find imageshack.us is a fast image host.
This is an example of what the image looks like saved in .JPEG using paint.
Saved in .PNG
Saved using photoshop save in web...
