If you add the Attachment content type to your popup it will display your images inline (gallery or list view) but it also displays the filename underneath, which is messy:

It's possible work around this by following the instructions in this blog post to construct the attachment URL in ARCADE then add an 'Image' content type to your popup, but it would be so much nicer if there was an option in the Map Viewer allowing the user to display or hide the filename.
Further discussion here: https://community.esri.com/t5/arcgis-online-questions/is-it-possible-to-remove-the-attachment-name-f...