Why does adding custom image to visual require a URL?

Reason I ask is that my dashboard looks fine on QuickSight but when I email a report it says it could not fetch the data (image url).

Any ideas?

And if static images could be placed in reports that would help. In fact, it should be a feature to have a default corporate logo for all your dashboards kind of like a master slide in a presentation.

The behavior you have described is similar to the bug reported in this thread that we are actively working on.
Regarding your feature suggestion, we have noted that to be considered for a future roadmap item.