Hi Mark, the unsupported_image logic happens on the server, so the client itself shouldn't make a difference. It looks like you are using different format and size parameter values between the two though. Can you try mtaching those to see if that makes a difference?
Otherwise, are you sure it's using the same app, account, and path as the curl command?
Sorry about the tardy reply. Looked again at this over the weekend, and think I found the answer. The error message it would seem is linked to the size of the image file. It appears the HTTP interface either handles this differently or can handle larger images. Will continue testing tonight and try and come back with some more definative findings with different size images/resolutions.