Unfortunately, there is not a native feature in Grist that would show the actual image from a URL directly in a column. I would usually suggest the two options you already listed with the attachment column and Image Viewer custom widget.
This would be a very cool feature to add to Grist though. I’ll share this with our team as a future improvement idea!
In excel and google sheets there is an =IMAGE function to insert an image into a cell and I was hoping Grist had something similar. It would be great to add this as a feature.
I took a quick look, but couldn’t find an enhancement request process. How does a user request a new feature?