First, if you want to ignore those, use
[Tt]humbs.[dD][bB]
as the ignore pattern.
Second: when you use the global ignore pattern in the TSVN settings dialog, that change will take up to five minutes to take effect (because the value is cached).
Although, in your case ignoring won't work: Subversion never ignores files that are already under version control! (you mentioned here that the overlay icon changes to a red exclamation mark, which means that file is under version control). You have to first delete the file from SVN, commit and then ignore it.