First thing to do is check both ends of the lens and make sure that there are no fingerprints on the glass.
It is possible that you have inadvertently set an adjustment wrong. Check the manual and see if there is a "reset" you can do to restore the camera to factory settings.
Check several of the photos EXIF information that is embedded in the photo file. See if you see something that does not look right....such as the ISO set very high. Also see if the shutter speed is abnormally high or low (most daylight shots will be from 1/50th of a second to about 1/500th of a second.)
Also check the f-stops and see if is changing from one shot to another.
If all of that checks out....it is time to talk to Olympus.
Downloaded photos from the E-500 appear dark, murky, and gloomy.
After passing them through a photo program they are OK but not much more.
Thank you for your help.