I'm trying to download some pictures off of my camera and either on a flash drive, onto my computer, or even online, and whenever I try to do so the pictures become dull and dark. This is quite a problem for me considering that I'm going to be selling what I'm photographing. What's causing this and how can I prevent it from happening? Any help would be really appreciated.
Try to look at the histogram of the pictures. Is it unbalanced to the left? If so, you may be tricked by the preview screen.
Also, could it be due to the screen you are using, which has poor contrast or low backlighting? Note that, typically, laptop screens don't have much contrast unless you set the backlight high or if exposed to sunlight.