Hmm...I am wondering if this is a feature that is not supported in ArcGIS Online. As you see in the pop-up configuration, it wants a text field that contains a URL (or just a static URL), not one that contains raster data directly stored in the attribute (which I believe is a BLOB data type). If there is a way you can host them on an image server and get a URL that way, then they should display in the pop-up.
Also, I am curious, once you configured the pop-up to display the Picture field and saved that configuration, did that field appear in the attribute table in Map Viewer?
Best,
Katie
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