JPG (Joint Photographic Experts Group)
The JPG format supports a palette of over 16 million colors. This
allows for high quality photographic images, but that large a palette
could make for huge file sizes, which is unacceptable on the Web.
JPG takes care of this problem by compressing the files, but JPG's
compression involves loss of data. The more you compress the
image, the more quality loss you get. You need to experiment with
each file to see how much you can compress it before the quality
degrades too much. Still, it works wonderfully for photographic
For images that have less color complexity (charts, buttons, etc.), the
compression is less effective. There aren't as many places to "hide"
the quality loss...the images are too simple and the degradation of the
image too obvious.
GIF (Compuserve Graphics Image Format)
At the most, GIF images can be only 256 different colors, so right
away it looks like JPG has an advantage. But it is the fact that you
can use less colors that makes GIF attractive at times. With GIF you
can reduce the palette to suit the image. So, if your chorus has a logo
that has only 2 or 3 colors in it, you can reduce the number of colors
in the image's palette, and make the image much smaller - without
the degradation in quality because compression is not necessary.
I used this technique on some of the images used on this site. "The
Basics" as a JPG was 5k, while the 4 bit (16 color) GIF is only 1.6k -
with no quality loss. "Links" was 4.2k as a JPG and is less than 1k
as a 4 bit GIF.
Another advantage GIF has over JPG is its ability to do
transparencies. If you don't know what a transparency is, look at this
sample. You can obtain effects using GIF's transparency that you
cannot with JPG.
There is some controversy and confusion over how to pronounce it.
Some say it with a G sound, like "gift", and some use a J sound, like
"giraffe". I prefer to use the J sound, as the creator of the format used
the J sound and coined the joke, "Choosy programmers choose GIF."
For photographs use JPG - its larger palette and compression style
make it the best choice for photos.
If you want a special effect transparency, you'll obviously need to
For logos with few colors, try using GIF and reducing the number of
colors in the palette. By experimenting, you'll be able to reduce the
size and not lose any image quality at all.
For images with a larger number of colors (shading, color gradients,
etc.), try both. Experiment with different palette sizes with GIF and
different compression levels with JPG and see which gives you the
highest quality at the smallest size.
It's not a black and white choice. You simply must take a little time to
experiment with your particular images and see which works best.
But this basic knowledge of the formats should help lessen the
experimentation time, and increase the final quality of your images.