unfurl and guests not allwed to see pictures/thumbs

[Robert9]

There si a forum where guests are not allowed to see any pictures. No attached, no thumbs, no nothing.
Also the unfurl cant see anything. Instead of these all users have to see the ugly x for picture not found.

Can I show a dummy picture instead?
Can I manipulate unfurl for these case not to show any picture and also not to show the x?
unfurl and guests not allwed to see pictures/thumbs

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