It seems that a logo cannot be uploaded and fails with the following message.
"Logo could not be uploaded" with not mentioning that the file type is wrong.
An upload as PNG works.
(Firefox)

If Service desk requires this file type than it should be mentioned in the Error message or upload filed, otherwise jira administrator will fall into this issue and will have to try which file type will work if they get the idea to test the upload with a different file type.