First, You need a decent Image editing program. If you don't want to download one you can use
Pixlr. Pixlr works a little differently so follow these instructions.
For Gimp/Paint.net- Size the Image at 512x512.
- right click "Background" or any other name for the base layer and select "Add Alpha Channel"
- Use the Color Selector tool to delete the white layer.
Your image is now transparent and you can do whatever with it.
For Pixlr.com- Select "Create New Image"
- Size the Image at 512x512 and check off "Transparent"
Your image is again transparent and you can do whatever with it.