A picture is worth 1000 words... unless it is under exposed, then it is only worth 403 words (which is the word count of this blog).
Real estate professionals, unlike car salesmen, have an infinite number of sizes, styles, colors and models to choose from. So while a picture of a car in a newspaper ad for the local dealership might be important to show customers which model is on sale, a real estate listing shows THE property that is for sale. That is why the images must be good, because the customer is looking for THEIR house, not a similar house. You say you don't have any Photoshop skills, or your camera is old, or it was just too dark by the time you could take pictures? Well that is understandable, but it is still no excuse. You don't need to hire a professional to come in with their own lights and spend hundreds of dollars to get quality pictures. What you need is to follow a couple of rules and find someone that knows Photoshop well enough to adjust or color correct the pictures.
First off, if you use a digital camera go ahead and use the setting for the largest picture size. This will give the Photoshoper maximum digital information from which to fix the picture. You can always reduce but you don't want to digitally grow or stretch the picture because that will leave the image fuzzy or pixelated. Second, set the camera for the dark setting if you are inside, the ‘sun' setting if outside, and make sure that the picture is taken at the fastest shutter speed available. This will give you the best possible (in a general sense) pictures.
If you need your pictures adjusted you can find graphic artists on craigslist.org that can help. Adjusting and color correcting images is an art form, but there are techniques that are quick and simple and will yield great results with most pictures. If you are spending good money to have a picture(s) run in a magazine have a professional (not your typical craigslister) color correct them. Depending upon the number and severity you can expect to pay between $5 and $25 per photo for basic adjustments. Well worth it when you consider that you are selling a home for hundreds of thousands of dollars and the only way to attract people to it is with your pictures.