@sam I suspect @feron is using the Homey Image Viewer Custom Tile which would explain why the attribute is grayed out - presumably that attribute is not used in a dashboard or rule trigger elsewhere.
That being said, I put together a Media Image Display Custom Tile that can be pointed directly at a Thing Attribute (or Variable) with an image URL and displays it. This is a newer approach that takes advantage of the existing Homey connection to SharpTools so it can update in realtime and doesn’t require a separate in-browser connection to Homey to poll for image updates like the old custom tile did.
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Alternatively, if you want to troubleshoot the existing version, if you have a laptop or other computer, can you try opening a dashboard with that Custom Tile and see if there’s any errors reported in the Console of the browser’s Developer Tools? (How to Open Browser Console)