@NigelG is right. Since bubble apps are served over https, images are getting loaded only if they’re coming from a site on a similar protocol. Now the good news is that when you’re on your own domain, it’s not necessarily https, and hence will work.
I tried to look at the forum app but wasn’t able to. Can you describe how you got it to work. In particular, what is the type of the data field? Image or Text. When it’s an image, the interface only allows me to upload an image (not specify a URL). I tried making a text field and copying the text in but type checking rules seem to prevent the text field from being shown as the right hand side of an assignment in the “make changes to item” workflow step.
This works very well with the image field like Nigel says.
in the dynamic view you could for example catch it as a URL parameter or from your DB. You want to put it in the dynamic image field. The formatting would be https:[parameter here]