Always make a master file!
Always make a master file and run off copies for the web from the
master. If your graphics program has a native file format, use
that. If your graphics program doesn't have a native file format,
save your image as a full-color PNG and use that as a master file.
See our section on "JPEG rot" to find
out why your master file should never be JPEG file.
Lossless or lossy compression?
Except when you're doing what you need to do to avoid "JPEG rot", forget about "lossy" versus
"lossless" compression. When you turn an image into a GIF file, you
reduce the number of colors to 256, and you can't get the lost colors
back. That's a huge loss (if you happened to do it on the wrong kind
of image), and if you use the default or "web safe"
palette for GIFs, you get even fewer colors, and chances are, the
colors will be wrong.
Lossless means that what you get out when you decompress is identical
bit for bit to what you had before you compressed. There's a big
difference between that and "very good quality" or even "visually
indistinguishable". Most of the time you don't need every bit. A lot
of the time (when you reduce the image size) you don't even
want every bit.
What you care about is quality, which is nothing more complicated
than (a) how it looks, and (b) how much bang you'll get out of every
Some web hosts charge you extra if you have too much bandwidth on
your site, so the time you spend making your images just the right
size will put (or keep) money in your pocket. Even if they don't
charge extra, a site with lots of overweight graphics files will take
so long to load that your visitors will give up and go somewhere else
where the site owner was more considerate. Remember, your website may
be speedy, your visitors may have broadband, but all it takes is
one slow point between the site and the visitor to separate
the speedy, successful websites from the slow, lonely hogs.
You can control the quality of JPEGs, usually with a little slider.
Some graphics programs and third party tools let you see the results
while you play with the parameters. If you play with the controls
for GIF and PNG, you can change their size and quality too.
Until you're dead sure when you should save a JPEG, and GIF, and a
PNG, save all three. It'll take you 30 seconds, tops. Then look
at the size of all three files, look at the quality, and make up your
mind which file you want to use.
Remember: quality isn't a number. The best tool you have
for judging image quality is your own eyeballs. Well, best except
for one: the very best tool you have is your experience.
And you get that for free! Every time you make another image, you
get a little smarter.
10 Second Summary
"JPEG" stands for "Joint Photographic Experts Group". So if
your image is mostly like a photograph, save it as a JPEG. If it's
mostly like something else (text, clip art, engineering drawings,
scans) save it as a GIF or PNG. When in doubt, save it as all three,
look at the size, look at the quality, and make up your own mind. And
always make a master file. That's all there is. You can do this.