Why not to use gamma correction
The first I heard about the term gamma correction was when I was first introduced to website creation. Back then I didn't create any images, nor had any need to publish any on the web, and so I subsequently forgot all about it. Now that I'm uploading a few photos to my site I have needed to revisit the issue.
The output brightness of a computer monitor is not normally linear. For example when displaying a grey square, with a colour half way between white and black, the result will not be half as bright as a full white. This effect doesn't just apply to the brightness of greys, but also the shade of most colours, and so can completely change the viewed image.
Gamma correction is a factor by which software can correct for this non-linearity, so resulting in a correctly displayed image. A longer and more complete explanation is offered on this Gamma Correction Explained page.
As an asside from gamma correction: it is worth adjusting the contrast and brightness of your monitor to get the best images. The best explanation I've found on how to do this is on the Accurate Image Manipulation site.
The average CRT monitor requires a gamma correction factor of approximately 2.21 to display colours correctly. Almost all PCs (whether running Linux, BSD, Windows or anything else) have no gamma correction applied to the signal which is sent to the monitor. Apple's machines have a gamma correction of 1.4 applied to their output signal (which is why pictures on a Mac are brighter than on a PC), and require a further correction of 1.57 to bring the true colours out on a CRT monitor.
With most computer systems using the web being un-corrected, most images created for distribution on the web have been (consciously or otherwise) adjusted for display on an un-corrected monitor.
To complicate matters further (and this threw me for a couple of days) most digital cameras, scanners and printers now use the sRGB colour space by default. The main signifcance of the sRGB colour space is that it assumes a gamma correction of 2.2, which means that images in the sRGB colour space will display correctly on an un-corrected monitor.
When editing of photos from most digital cameras there is no need to alter the gamma correction settings of your computer, because the image will already be in the sRGB colour space.