Hi,
If your photo has been rejected because it is too dark/underexposed, then without seeming to be rude the logical answer would be to lighten it up. This can easily be done these days and a simple method is to use Windos Live Photo Viewer, where you have the basic tools for size,exposure and color adjustment.
If the problem is in the taking, then the answer would depend on what you are taking it with, Mobile phone or digital SLR. If a mobile phone then do not be fooled by all the hype about whch has the most megapixels, that is all about size and nothing to do with the quality of the picture you will end up with. Maybe you have a personal friend with a digital SLR who could assist you. There can also be a mismatch in how the same picture looks when displayed on a phone or computer, may times when i wish to send a picture over my mobile i have to adjust the brightness because it just does not look the same as it did on my computer.