Here is a video tutorial of how to upload your own background images to the flyers.
Video Tutorial for Uploading Background Images
Note some of the following tips:
Some users may want to create a letter head type background that places a few custom graphics in various places on the flyer.
The images you choose for the backgrounds can be of any size. When you upload the graphic you then will be presented with the option of cropping out the portion you want to keep. The resulting background will be saved and resized to 576px × 719px. This ends up being a letter sized 8.5 x 11 inch image.
Since the image will be resized, it is not meant to show at high resolution so if you have very detailed type setting embedded in the image, you may not get the expected print quality you need.
For high resolution type setting, it's best to use the free form type text fields that can be added to your flyer. You can add as many as you need to label any part of the flyer with the added benefit of using different fonts and font colors.