Hi Lis_stanger,
Sorry for the delay in response.
The pdf form you are referring above, Is it a Adobe Sign or created in Acrobat DC?
The functionalities of a desktop app and mobile app are quite different. You do not get access to all the features in mobile as compare to desktop.
As your clients are trying to add an image to the form field in the PDF form using mobile app. Sorry to say, that this functionality is not supported on the mobile application. They can do that using desktop application but it isn't supported in Adobe Acrobat Reader mobile app.
You may try the workaround provided in this forum link How to insert a photo into an image field within an acrobat form on an iPad.
Hope this answers your query.
Regards,
Akanchha