Actually, I am not sure that GIF is the smallest image size.
We found that PNG images (256 color, highest compression
which is lossless) are generally 50% to 80% smaller than the GIF
images.
You need to be aware of the risks of using PNG images.
Mostly, save a back-up of the original images, so that you can
revert to them if you do discover a problem with crashes and/or
display problems -- we never had any troubles with PNG.
We use IrfanView for batch conversion to PNG (don't forget to
downsample to 256 colors and choose compression = maximum). And we
use the RoboHelp multiple-search/replace tool to replace the file
name extension in the RoboHelp topics.
This worked like a charm.
We also experimented with different graphics software, but
for some reason, Irfanview produces the smallest yet most
compatible PNG images.
In Irfanview, there is also the option to use the plug-in
PNGOUT, which reduces the PNG file size a lot, but RoboHelp cannot
display all of these images. Once day, when we get a chance, we
will convert all our images with PNGOUT, and then replace the
"broken" ones with their original PNG images.
Finally: All of the above is valid for screen-shots, plots,
etc. but not for photographs. Nothing really beats JPG for that
purpose, but experiment with the (lossy) compression. We use 50%,
and our photos look just like their originals, but the files are
some 10% of their original size.
Good luck.