How do I convert an image?

[Frodo]

I copied the 'Mark Warner for Senate' banner image from his campaign website to the 'My Pictures' file on my computer, and edited it.  My object was to then post it in the gallery section here so that when I do put it in my sig, that it won't suddenly disappear on me.

Well, the image happens to be a bitmap (bmp), which this site cannot accept unless I convert it to a 'jpg'.

So, how do I convert an image so that I could post it in the gallery?  Evidently I can't just change the last three letters of the image, which is what I tried to do.