The problem is most likely involving the naming of the file.
https://www.zen-cart.com/content.php...s-to-a-product talks about how to name files for additional images.
Basically, if your main image is vinegar.jpg, naming the extras vinegar_01.jpg, vinegar_02.jpg, etc will automatically present the extra images.
There is an app called GoodReader that is supposed to give quite a bit of file management to the iPad including renaming images. It's a paid app so I have not tried it.
Another option is to use image handler. Image handler allows you to set any file as an additional image. With 1.5.4, you will need to find version 4.3.4 on github.
The advantage to the app would be that you could rename all the files and FTP them to the site in one swoop. Then the system would automatically put them where they belong. NOTE: Be sure to read the link fully if you decide to use the renaming app. Similar file names can get you in trouble by adding them to the wrong product. It's discussed in the link.
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