No, except of course if the file ends up larger. In the ancient days of the
web (let's say, back in 96-97), it used to be an issue because JPGs are a
compressed file format. They always have some level of compression on them
so the computer had to perform some more calculations than on an
uncompressed gif. Back then computers had less horsepower than my cat so it
was sometimes an issue. Nowadays though the additional calculations aren't
even a blip on the computer's workload.
As far as converting the jpgs to gifs, you'll need a third-party graphics
application to do that. You can search tucows.com for some decent format
converters as well as sourceforge.net for some opensource versions. I'm
sorry I don't have any specific converters in mind though because I haven't
had to look for one in a long time.