Here is a trick. Don't worry about pixels. Use photo software like Photoimpact.
You can enlarge the size of the picture so it takes up more space than eBay gives you.
What you do is make the photo transparent. Meaning you use the software to eliminate the background or make it clear so the background comes through.
Then past it on another background but make it square. 400x400. Make the background any color you want blue white whatever.
But before you paste the transparent image make it 450x450 or bigger.
So when you paste the image onto your background it is larger inside the new background.
Now when you upload it to eBay the item you are selling takes up a lot more space inside the square eBay gives you.
Your item will stand out because it is bigger. It draws more attention and almost appears like it is a special listing.
You can also write on the pic using Photoimpact. You could say BEST or BEST DEAL. And another trick I shouldn't give because it is pretty kool is this.
Now this takes some work to learn and you will spend several hours learning this but you can actually create a colored strip around your picture.
So now your pic will be extra large and have a colored edge. I have made picture look like they were glowing using this trick and photoimpact.