https://en.wikipedia.org/wiki/Design_rule_for_Camera_File_sy...
TFA, "Apple uses the ‘IMG_XXXX’ naming convention for all images and videos captured on iOS devices, where XXXX is a unique sequence number." isn't very accurate, as the numbers are not unique. They're just sequential. if you take 1001 images, the file system will actually create a new folder and roll the digits back to 0000 to avoid overwriting