I searched the threads and didn't seem to find anything related to the subject so apologies if I somehow missed it.

My post relates to quality degradation of my digital photos. Some of my photos aren't always taken in the best settings as it's not always possible to adjust my settings before snapping. Mostly it's because of cloudy skies or general darkness. I'd usually adjust the settings afterward with a photo editing program and save the new photo to my hard drive. I'll notice when I'm looking back through my archives that the photos seem darker than they were when I originally saved it. Once a photo is edited and saved how can it change this way? I've seen it happen to pics I've saved to my own PC and to those I've uploaded online. If I keep having to re-edit these photos time and again I'm losing the original resolution. Can anyone explain how this happens?

Have you gotten a new monitor or changed the settings on your monitor. That might account for the changes.

Some photo formats do lose quality. Jpeg, which is what most people save as, is. Then there are other formats, such as Tiff that are lossless. They're usually very large files. How you plan to use the photos usually determines how you save it.
I normally save things in more than one format, one jpeg that I can play with and email and such, another for permanent storage that I know won't look crummy after playing with it.
Here's one article that explains formats; there are loads of articles out there, just search on "digital photo formats" or something like that if you want more details.
Photo formats

Thanks for the tip. I do save everything in JPEG form. I have reopened some of my affected files and re-edited them. I did save them as TIFF file and I see what you meant about the file size - it's huge! At least now I will have them in good quality forever - Thanks again for that...

Good tip Taesma. If the file size is of concern, there are a couple of ways of reducing it. Your photo editing program may give an option to compress a tiff using LZW compression. This compresses the file size to about a third (or less) of the uncompressed file size without causing loss of quality on editing. If your editing program has this option, the compressed file will open as soon the file icon is clicked. A drawback might be that if you were to send or give a image file compressed in this fashion to someone else, their editing/viewing program may not open it (Irfanview, a free image viewer/editor that many use would open it OK). Alternatively, a zip program should compress a tiff by approximately the same amount. This would mean unzipping the file(s) before they could be viewed but it is a good way of storing photos if file sizes, or disk space, is important.

PNG is another lossless file format that keeps files sizes down while not causing a decrease in quality on subsequent edits. It keeps files to near the same sizes as compressed tiffs. It is also viewable in browsers and most image viewer/editing programs.