If you upload a JPEG or a PNG with a background, that colour will be treated as part of your design, not as transparent (meaning it will print and could look like an unwanted 'blob'). To avoid this, you need to REMOVE the background (whatever its colour) from your image and save the result as a PNG with transparency.

When you upload an image to the personaliser, you'll see an option to choose whether you want to automatically remove the background or continue with the file exactly as you've uploaded it.

Here's a step-by-step guide on how the process works from the website:

If you choose to remove the image background, a file with a transparent background will be created automatically. If the change looks right, select "use image" to carry on.

To undo this effect, simply deselect the "remove image background" option.

It will appear directly on the modified T-shirt.

You can see how it works in this example: