Looks like @ambroisedlg identified the issue for you. 
Technically, what was happening behind the scenes is that Bubble was auto-converting the list of images into a comma-delimited list of URLs and using the resulting string as the URL for a single image. Obviously, it is an invalid URL, so no image appears.
Since you’re dealing with lots of images in your app… If you haven’t already, you might want to devote some thought as to how best to manage the uploaded image files. If your app gains traction, you could potentially chew through your [pricey] Bubble storage rather quickly - especially if users upload lots of “hi-res” multi-megabyte images from their mobile devices.
It’s important to realize that simply deleting a post, will not automatically delete the files associated with that post from your Bubble storage. You must explicitly delete the files in order to reclaim that storage capacity and prevent “orphaned” files. This is also a consideration if you’re allowing users to edit existing posts - i.e. add/remove pictures. This thread might be helpful.
Additionally, automatically downsizing images before they’re uploaded can dramatically cut down on storage requirements. That’s exactly what Upload Buddy does.
Regards,
-Steve