Hi Alex,
If I understand you correctly, the feature service resides in the Enterprise environment and is registered (with credentials embedded) in ArcGIS Online, correct?
If so, there are a couple of comments:
- Unfortunately, the images will not work in the report functionality. This is due to a limitation with how ArcGIS Enterprise geodatabases currently store attachments; they do not have the ability to store the additional metadata needed to be able to associate an attachment to a particular question.
- We have had other reports of issues submitting attachments with services that are being proxied by ArcGIS Online; this is being looked into.