Lost token image

In my campaign Awesome-O I had lore put into the Gm notes of question mark tokens, the imagesrc were on a website which no longer has it. My tokens now have the image not found thingy in my library, but in the gm layer I can select them with box select. On the campaign map however they have no source image so they are invisible. I also don't know how to replace the image source of the token.

The fact that the image is not available on an external website won't affect the tokens in your game. When you drag an image into Roll20, it uploads it to your user storage, so it does not depend on the external file anymore. However, if you delete the image from your user library, it will be broken as you describe (that is why there is a large popup that occurs when you go to delete it). Unfortunately, there isn't an easy way to replace the image of a token without having access to the API (A Mentor Level Perk). I'll highlight your issue to the devs, but your best course of action is likely to select them as you mentioned, then double click them and copy the text to another token.