On the Create New Release page:
As an experiment, I tried to upload album artwork that was far too large. No notification appeared to explain why the album artwork wasn’t accepted.
I did eventually find an notification by hovering my mouse over the [Upload] button. A hover message said “File not supported”. This is much too subtle, and as a user, there is no direction for what I need to do differently – no direction for how my file should be different.
