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:
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.