It looks like customizing the image is now only available for Enterprise plans. I was having the same issue and they mentioned this changed about a month ago.

From Branch Support:

This was changed after some privacy concerns were discovered.

At this time, only customers on an Enterprise plan have access to edit the link image preview. This was quite a surprise to us on the sales team, and I understand that it may be disappointing and frustrating.

Most customers upgrade to Enterprise once their app has 100-150K+ MAUs and/or they're ready to make a significant investment in deep linking and attribution (i.e., Enterprise plans start at $36K per year).

Again, happy to connect and chat further - please share any thoughts or questions in the meantime.

Best,