I use Photoshop for my images, but there is free software available such as GIMP and I think you can get old versions of Photoshop for free now.
This is my original image; it looks ok, but there is definitely room for improvement.
If you don't have a white background, then the best thing to do here is to go into Image > Adjustments > Brightness/Contrast. Up the brightness by about 60, and lower it until you find the point it looks best at; I find if you start at the bottom and go up, you tend to stop before it's really at optimum brightness! Do this again for contrast, this will bring some of the depth back which was taken out by brightening the whole image. It probably won't need to be much higher than 20, but play about with it until you are happy.
Laura x