If your design has a white background, but you want it to have a transparent background, the issue is likely the design file or the file format.
Step 1: Make you're using a PNG file. #
The most common cause of non-transparent backgrounds is using an image format other than PNG. Be sure the design you exported from your graphics software is a PNG with transparency enabled. The JPG file format does not have the ability to save transparency. A transparent image saved as JPEG will show as pure white.
Step 2: Check the original design. #
You'll want to make sure the design you're using was created with a transparent background in your graphics software. Wherever you created your initial design (Photoshop, Illustrator, online design tools, etc.) you'll want to be sure you created the design with a specifically transparent background.