Ignore the previous posts. (GIF and JPG are the only file formats supported
by FP or browsers, my granny's wrinkled brown eye!)
Be aware, I had a very similar problem, and I had to reinstall FP. Before
being so drastic, try the FP cleaner application to see if you have some
garbage files in the FP cache, etc. Do a search for "FPCleaner.exe" (I got
sent a copy by someone on this forum, but my post has been deleted and I
can't find the link, sorry!)
If that doesn't work, try doing a repair (in Add/Remove applications, click
the "Change" button in the Frontpage entry). If that doesn't work, think
about a reinstall.
Now. Despite what the previous posters have (somewhat dubiously) stated, you
should NEVER, EVER use GIF images unless you're desperate and possibly rich,
or you can't figure out how to manage transparency with PNGs or other
formats. Do a search for "GIF" and "extortion" and you'll see what I mean!
FP handles nearly all common graphics formats, including PNG, BMP, and the
Microsoft formats (WBM, etc). So if you can live without GIFs, do it. PNG and
MNG graphics formats are kinda new, and really really old browsers don't know
how to handle them, but if you're developing a new website, you probably
don't want the hassle of backwards-compatible-ing the site to allow IE3 and
Netscape 1.0 browsers to work (I've spent hundreds of hours getting websites
to work with old browsers, and in every case, the ROI was just not worth the
hassle and annoyance).
Since FP *should* handle the existing images, try loading the images in
another graphics editor (Photoshop or something, even Paint can find and fix
problems!) and save them as a new file and try loading that in FP.
If you do use Photoshop, be aware that "Save As" and "Save As Web" do
different things. Also, be sure you use the smallest colour depth possible
for the image; if it looks fine in 8-bit mode, use that, as FP does get its
knickers in a twist if Photoshop saves as 16-bit.
See how that goes. Be aware, though, that I had to reinstall FP completely.
On the upside, that did fix all my image format problems!
Good luck!