If you upload a JPEG or PNG with a background, the colour is treated as such, not as transparent (which means it is printed and it will not look good).

To avoid this inconvenience, you must REMOVE the background (whatever the colour is) from the image, and SAVE the result as a PNG with transparency.

When uploading an image to the customiser, you will see the option to choose if you want to remove the background automatically, or continue with the same image you have uploaded.

Here is the link to the FAQ in case you need to see the context: If I upload an image with a background, is the background also printed?

For more detailed instructions on how to do this, click here.