You can do this using an image picker group. Follow these instructions:
1) Create a field on the page of your master pdf to which you would like to have background options. The field should be the size of the entire page.
2) Upload the master pdf and commit changes
3) In the Assets area of the template, you would upload an image for each of the background options. Upload these to an image picker group.
4) On the Field Setup tab of the template, have the field set to be an image field coming from user entry.
5) On the Data Capture tab of that field, assign the image picker group containing the background options.
6) Make sure that on the Format tab of the field, you set the stacking order to be lower than that of any fields that need to print on top of the background image.
7) Save and commit changes.