It does work and I think you are not understanding why I am saying run that upgrade. For the feature to store a url in edit categories your thumbnail column must be varchar not int as it was pre 3.0
Code:
Content visible to verified customers only.
This query is right in the upgrade script for 2.6 to 3.0 and it changes the field I noted to varchar which would be correct for this to work.
Is your url more than 125 characters?