So you took some nice photos, edited them with your favourite Linux photo editor and published them on the web. But then you show your pics to a friend who is running Windows and IE 5 and suddenly your photos look very different. What happened?
Your monitor colors could be different than the one of your friend. This can be corrected calibrating the monitors with a colorimeter and e.g. Argyll.
Most photos have a color profile embedded. This color profile defines, where the rgb values are in the colorspace. Some software is aware of it. Some is not.
Check out this color profile browser test (german only). Especially this picture:
or this one:
Color management in Firefox 3 has to be turned on and if you test various GNOME/KDE picture viewer and editors, you will notice that only some of them cope with color management.