<not sure if this is your issue> The (source) file name is absurdly long (and has spaces)
a 'cover' has 2 parts:
The HTML page it is on , that part seems to be there
or you would not see the broken image icon
the image file (jpg, png...). most likely, this is missing or the path name is in error. It is case sensitive, (many devices) abhors spaces in the file name
If you had used the tool with the calibre editor, this should have been fine.
(editor) Tools: Add Cover:<choose>
The cover can be huge (anything over 800x600 is huge to my devices
), as long as it is a valid format