When you say load from an external source, how? Through an SD card adapter? That’s how I tend to bring in a lot of stuff from devices
other than my iPhone. Within the Files app, you just need to make sure that you save the stuff to the iPad and not the iCloud. You can create your own folders. From here it’s pretty easy to save to the Camera Roll providing it’s an image.
When you say the apps can’t find the images in the Camera Roll, what type are they (eg png, jpeg)? Might they be raw in which case only certain apps will open them. Also, sometimes you have to make sure you have added the option in the apps Share menu. Not always straightforward I know. I have often been frustrated by apps that don’t give you the correct import/export options and have had to find another route.
Or there’s also Transfer which I tend to use less now unless backing up which is a lot quicker than the Files app.
Photo Transfer allows you to easily send and share photos and videos through WiFi transfer. Make a backup before transferring photos and videos quickly and move your files from other mobile devices to iOS, iPad or computer and vice versa. With over 2,5M downloads, more than 9k reviews in the...
apps.apple.com
Note: if you don’t buy the app then it won’t transfer photos at full resolution. It’s quite expensive now. I paid about £3.99 for it some years back. Now I think it’s £19.99 for lifetime.