The only way I know of to do that is to convert the image file type within a graphics program. Getting a graphics program is the hardest part, some popular options and free versions are:

Paint Shop Pro (currently in version X) used to be made by JASC, now made by graphics giant Corel. A 30 day free trial version can be downloaded at: Corel's free trial site.

ULead Photo Impact, also made by Corel now and also available at same free trial link above.

Image converter and resizer: full list of free graphics file type converters and resizers available here: best shareware graphic converters.

List of free graphics software I found thru google. free graphics.

Now, once you download and install the graphics program of your choice, and assuming you have all these jpegs you want to convert saved on your harddrive somewhere..
locate and open up each JPEG you want to convert.
Go up to File and down to Save As. Save it instead as a .gif
Name it something different when you do to make it easier on you!
Upload new images here at AW and replace the old. That's it! Good luck.