A good picture is worth anywhere from a 10% to a 200% increase in the final bid and can be the difference between a sale and no sale.
A picture that takes too long to load can be as bad as no picture at all. Remember part of selling is maintaining the positive attitude of the prospective buyer. If they get irked, they won't buy. As a rule of thumb, every K (kilobyte) takes about a second to load with the average modem. If the picture is larger than 30K, it's probably too big. (There are always exceptions, but no one using a modem is going to wait for a 200K picture to load. At least not without counting it as a mark against the seller.)
Choose your background carefully. A solid color background will not detract from what you're selling and will compress better. This can make the picture's file smaller without you having to do anything else. Avoid mottled backgrounds like rugs. The variation will maximize the storage required to save that part of the image and it can only distract the viewer.
Crop out what you don't need. There are a variety of image processing software products available over the counter and as shareware. A good one will allow you to easily crop, enhance, and adjust your image. A few moments spent making the most of your digital image can make a big difference in your profit. Remember that screen resolutions vary from one computer to another. The most common screen resolution for PCs these days is 800 x 600. If your machine has a higher resolution, your image will display much larger and may have a much poorer resolution on your prospective buyers' screens. A small, sharp picture will do you more good.
Be aware that there can be a big difference between the brightnesses displayed by various monitors on PCs and MACs. The best advice is to get feedback from people with different equipment than your own and adjust accordingly.
Don't spare the hyperbole, but back it up with your product and service. Remember that the best business is return business. A good way to lose a customer for life is to sell them something that doesn't live up to their expectations. Words like "new", "exciting", "unique", "fascinating", and "amazing" are great sales tools, but don't use phrases like "museum quality" if it's really "kitchen magnet quality". You may make one sale, but you'll lose that customer forever.
Be kind and considerate to your customers. Even if you've intentionally ripped the customer off on this sale, a few kind words and a courteous attitude can save the sale or at least not alienate the customer. Avoid snide responses to queries from customers. There are a couple of auction sellers who will never see any of my money again, because they took a snide tone in response to my emails about the products I bought from them.
Don't hide the warts. A really good way to alienate a customer is to show them only the good side of what you're selling and not tell them about what's wrong with it. One seller sold me a fossil where they only had photographed the part that was in good shape. The other end was crushed. I wasn't surprised considering the opening bid, but I was disappointed.