Keep in mind, unless you've transferred the high-resolution files from the microSD card in the drone to your iPhone, the files on your iPhone are low-resolution cache files. You'll want to transfer the files from the drone itself, either by plugging the drone into your computer with a microUSB cable, or by removing the microUSB card from the drone and inserting it into a card reader connected to your computer. Then just delete the cache files on your iPhone.