Why is there a white box around my image? Will it show?

Short answer: Yes. Use a PNG instead.

Full answer:

If you upload an image with a background, it will appear on your shirt with a coloured or white box around it. Onto white t shirts, upto A3 in size, a white background will not show. Onto white polos or all coloured garments, whatever the size, this white or coloured surround will show.

To avoid printing the background, you will need to upload an image with a transparent background.

What you see in the basket is what will be printed.