IThmb
From Just Solve the File Format Problem
iThmb is a format used in iOS to store thumbnail versions of images (such as in the photo library). Pictures are converted to this format when imported into devices such as iPhones and iPads, and are intended and optimized for viewing on those devices and nowhere else, but users sometimes seek a means of exporting them to other systems if they have pictures on an iOS device and no longer have other copies of them elsewhere.