Converting Files

Im not sure where to post this, but its really confusing, so try to keep up

1. A while back I had a picture from the internet, and I saved it as my album artwork for a band.

2. I reformated my computer and lost the original picture, but it stayed as my album artwork for that band.

3. I cant find where I got the original picture, so I decided to get the picture from the album artwork, drag it onto my desktop, and continue easily from there.

4. However, when I dragged it on, the picture icon looked different, and when I open it, it doesnt open in preview, it opens in some unknown picture application, but no application opens in the dock or the top taskbar.

5. So I want to convert the picture to a .jpg file so I can open it in preview and be on my way.

So can anyone tell me how to succesfully change the picture to a .jpg file.

P.S. By the way I clicked "Get Info" and tried changing the extension to .jpg, but that didnt work.

Any suggestions would be great!