Is it possible to have an image attachment displayed as a thumbnail in an ArcGIS Online pop-up?

Answer

In ArcGIS Online, the current pop-up configuration supports links for attachments as opposed to embedding directly in the pop-up. By default, it is not possible to have an image attachment displayed as a thumbnail in a pop-up of a web map.

To display an image attachment as a thumbnail in the pop-up, the URL where the attachment is stored in the service must be referenced directly from the pop-up as a photo. This can be accomplished through the following steps:

Add the URLs of the image attachments as attributes in a field

1. Create a String field in the attribute table of the feature service through the Map Viewer.
2. Click on a feature to bring up the pop-up with the attachment link: Right-click the image attachment link and copy the link address.
3. Go back to the Map Viewer and Edit the feature. Paste the URL in the field created in Step 1.
4. Repeat these steps for all desired attachments.
5. Configure pop-ups on the layer to Show Images.
6. Optionally uncheck the box for 'Show feature attachments as links' in the pop-up configuration.

Automate the process by using a script to convert the feature service attachment URLs to a field

Note:
This option requires the use of ArcGIS Desktop.

Feature service image attachments can be displayed in a web map's pop-up using the workflow and script provided in the Esri Community thread Show attachments in web map pop-up. The script converts the feature service attachment URLs to field attributes. The attachment can be displayed directly in the pop-up as a thumbnail after using the workflow to Show Images when configuring the pop-up.