~(*mockup*) would fine all names that don't contain "mockup". Combine that with the file type filter at the bottom of the Tools > Find FIles dialog and you can get all JPG files that don't contain the word.
Thanks for response Leo --
I must be doing something wrong...When I type ~(*mockup*) on the top right search bar, it shows all of the files with mockup -- ( the opposite to the desired behavior)
The top-right search field sends the query to Windows Search, the same thing that File Explorer uses. It's mainly there to provide continuity for people moving to Opus from File Explorer, although it seems nobody knows how to use it in either program.
It has its own query syntax which is (not very well) documented on Microsoft's site:
On the other hand, Tools > Find Files uses Opus's own find functionality and is a lot more straightforward.