Deleted .tga From Regedit

That's not a file, that's a registry key. In this specific situation I doubt there are any consequences for deleting that entry but you really should be more careful in the registry as deleting the wrong thing can destroy your system. If you want to restore it, use either the registry backup you made before removing the key or run System Restore.

You also don't need to disassociate a file extension; I know, I know many people are convinced they do...but they don't. If you want to open the file in another program then right click on the file, choose Open With then Choose Another App, select or browse to the program you want using the More Apps button, and click the Always Use This App box to always use the newly selected program. Programs that call on files internally don't rely on file associations, file associations are for humans who want to double click on a file and have a specific program open.