This will happen for a variety of reasons. Most of these issues will not be seen until you continue to the online Proof page.
The most common reason is that the template has been cropped and is not sized to center justify your image.
To avoid this, you'll want to make sure that you upload the entire template (including all of the white space surrounding the template!)
Here are some other tips:
- Make sure that you are using the correct template
- Confirm that you have set up your design so that the background image is pulled all the way into the outer most crop marks to ensure that you have enough "bleed" area.
- Do not crop the template size (at all!) It's ok to upload even the legend on the bottom of the template.
- Save the file as a PDF. Do not PRINT as a PDF as it will change the size and dimensions of the template to the size of a sheet of paper - 8 1/2 x 11.