Explorer Online downloads the entire image and displays it as a thumbnail. Ideally, we'd generate thumbnails of uploaded photos so they can be viewed without a large download. Presently, arcgis.com requires a download to view the image.
Where is your feature service running? Is it a hosted feature service or ArcGIS Server feature service? One point worth mentioning is that attachments to feature services are stored as blobs in the database. If you're using a hosted feature service, the current credit consumption rate is 2.4 credits per 10 MB per month. Typically, the geometry of features doesn't take up that much space. However, when you start adding photo attachments, you can quickly enlarge the amount of database storage you consume. Consider many mobile devices take pictures at 2MB in size or larger.
thanks,
Mike