In order to display images stored in the SD/Micro SD card or any USB storage device, we have to let the iOS recognize it as a memory card from camera.
First, the DCIM folder must be created
Second, a sub-folder has to be created under the DCIM too. The name of sub-folder is minimum 8 digits. First 3 digits has to be in numbers and the last 5 digits in alphabet (either lower or uppercase). For examples, 100HYPER or 100hyper or 100Hyper, it is all fine.
Third, all images must be stored or saved inside the sub-folder.
Lastly, the image file is in minimum 8 digits as well. First 3 digits must start with IMG and the last 5 digits has to be in numbers ONLY. For examples, IMG00001, IMG11111
It won't work if there is any alphabet mixed with the numbers, for examples IMG0001A or IMG001test
It is fine to put an underscore in between the IMG and the last 5 digits numbers. For examples, IMG_11111 or IMG_00000
This is part of the iOS agreement to prompt the PHOTO IMPORT.