We supports both JPEG and PNG files. So you can upload either file format. But we recommend you to upload your designs in PNG format only because PNG files comes with a transparent background whereas JPG file comes with a non-transparent background.
Avoid using background unless it is a part of your design. It is especially important for the designs you want to print on a t-shirt or any apparel. Why? Because if a product you want to print on is the same color as the background of your design, it might not look as sharp once printed.
Let’s suppose that you decided to print a design on a white t-shirt. If your design has a white background, it is likely that the shades of white in the printed design and the garment won’t match and your final product may look odd (check the image on the left below).
PNG file doesn’t come with a background. It supports a transparent background which makes it easier for your design to be printed on almost any background.