There are two principal methods to transfer pictures from your camera(s) to your iPad:
1) If your camera(s) have a USB connection, you can use a direct cable transfer with a suitable USB cable and an Adpater; to do so, the camera must be in MTP mode.
- Lightning to USB 3 Camera Adapter
https://store.apple.com/uk/xc/product/MK0W2ZM/A
2) Copy photos from the cameras SD Storage Card to the iPad - using and SD Card adapter:
- Lightning to SD Card Camera Reader
https://store.apple.com/uk/xc/product/MJYT2ZM/A
Start here:
Connect external storage devices to iPad - Apple Support
Direct import of photos from your camera SD Card to your Photo library is possible - subject to a couple of caveats:
- the SD Card must be correctly formatted as FAT, FAT32 or exFAT (FAT64)
- stored photos are stored within the expected DCIM folder
Import photos using Apple camera adapters - Apple Support
If you are unable to use the direct photo import feature, you can still manually access and copy photos from the SD Card using the iPad’s built-in Files App. To do so, just open the Files App - and in the Sidebar you will see your SD Card in the list of Locations. If necessary, open the sidebar by tapping the sidebar icon at top-left of the Files App Window. Photos can be copied to the iPad Photo Library, or copied to a folder of your choice either on your iPad or elsewhere.