Use (Save as) in any picture viewer. I use Irfanview. (free download from the internet) When it brings up the format selector and name window just select .jpg format and name it whatever you want. http://download.com.com/3000-2192-10021962.html
Yea, but Irfanview is FREE, fast, and only takes up 817k. One of the few programs out there that is actually free. No popups/banners/time limits or other bullshit. And you don't need training to use it. Oh, and did I mention it's FREE!
Help for those using Irfanview to change to JPG and resize pics to fit on the HAMB. I usually resize to 640 by 480 and when you open up save as, hit the options button on the save as menu and select 80% on the slider. That'll usually make the file 70k or smaller to fit in a HAMB message limit while not turning it into a thumbnail.