Background image is the product images / uploaded image / blank image on what perosnalization will be appeared. By default there is a list of your product images and a blank image to use. You can add image from shopify admin as product / variant image or upload image on app by the right side plus icon.
Customer’s personalized png/pdf will be same resolution of the backgound image. Apps loading time and UX depends upon the background image size/resolution .
For Multiple views like Front, Back, Left, Right, Top, Bottom or whatever you just need to change the background image for respective field.
Note : The app does not store product images that are from shopify server. Please update configuration if you remove/rename/update product image from shopify server that are used as background image