There is no way to configure a default image (e.g., upload photo option) for an Image widget when there is no selection. Since the widget is set up using Selected features → Dynamic → Attachment, I had configure a "View for empty selection" query to a dummy record that had the desired placeholder image, which in this case was a product logo.
The dummy record had no geometry, only the attached image and an Object ID. This ended up breaking the "View for empty selection" query. It refused to render the image and continued displaying the default placeholder graphic of grey mountains and a sun (moon?). Testing multiple file formats (PNG, JPG, JPEG) yielded the exact same result.
The Fix: The issue resolved immediately after assigning point geometry to the placeholder feature since the record previously had null geometry.
Takeaway: If an Image widget uses an attachment from a "View for empty selection" feature, the underlying record must have valid geometry. Even though attachments are stored as non-spatial table data, Experience Builder appears to fail to render dynamic media if the parent feature lacks spatial attributes.
Solution: Allow users to just upload an image to an Image widget that is dynamically tied to a layer when no records are selected.
