When compressed to jpeg, my 3mp images are about 600k. If you look at the
file properties when the picture is opened (uncompressed) in an editor, it
does indeed occupy about 10-15 times as much memory, about 8MB. Don't worry,
it took me a while to figure it out. If you have the option to save as RAW
or TIFF format, you'll see they are much bigger. But for everyday purposes
the compressed files are fine, and conveniently small to send and store.
I've even had them in print.

Each decode / encode cycle introduces more errors, so if you are doing
major editing on a jpeg the safest way is to convert to a lossless
format first, do the editing and any intermediate saves in that format
and then only produce a jpeg for final use. If storage permits, keep the
"finished" picture in a lossless format. It is possible to data-reduce a
picture without loss of information, though obviously the saving isn't
as great; a program which saves TIFF usually has the option to apply
some form of lossless compression to that file.

Having said that, it is possible to perform some (simple) operations on
the image in jpeg format *without* performing a decode/encode. Examples
of things possible are changing the pixel density, adding and removing
comments, optimising the compression parameters, converting to
greyscale, converting to progressive format, 90 degree rotations,
horizontal, vertical and diagonal flips, cropping and changing the
contrast and colour balance.

There is a cross-platform standard utility to perform these operations
by the Independent JPEG group called "jpegtran". Its functions are
likely to be built into most processing software, but you'll have to
know the specifics of your system. I use RISCOS ports of jpegtran called
"JCUT", "JSIZE" and "JCLEAN" among others.

Try also:
http://www.faqs.org/faqs/jpeg-faq/
ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz

Hwyl!

M.