If you have less than 1GB of storage available on your iPad / iPhone, the Apple iOS software will start emptying apps' cache - the temporary storage space for docs and and other data. It is possible that images in ArtBinder get deleted during this Apple automated cleanup.
To avoid the issue of Apple deleting ArtBinder images when there is not a lot of room on the iPad / iPhone, please ensure you always have 1GB of storage available.
You can check and manage your storage in the device's general settings. Open the Settings app, click General, click iPad Storage.