Label of Custom label
From what I can see, each custom label has a user-defined value which persists even if the custom label is not used as part of the thumbnail label. This leads me to the following idea:
In Tools > Options > Browser > Thumbnail > Thumbnail label, the custom are listed as "Custom 1", "Custom 2", ...
Instead, they might be listed as "Custom 1: '<Size KB kb>', "Custom 2: '<Size KB> <Width> x <Heigh...'. This way the user could identify his/her custom labels more easily from the pool of his/her custom labels.
Label of Text field 'Custom'
The label of the text field for entering the custom label could display the number of the custom label (e.g. "Custom 1"). This might make handling a bit more intuitive (I'm not so sure about this one).
Size of Text field for custom label
The text field for entering the custom label (underneath the list of chosen labels) should be a bit wider. The text field plus the '>>' button could be as wide as the list of chosen labels.
