@Basher – If you're talking about the thumbnail in the Resource Manager, this is a bug that's been around for a couple years and unfortunately hasn't been addressed.   The bug occurs if you edit a texture while inside a Symbol (and perhaps other containers? can't remember). It also has something to do with the location of the symbol versus the document origin, but beyond that this bug is fairly predictable.   @Gunther I happen to know this bug is one you've entered into JIRA
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