I have my cams set to take a pic every 10 minutes. The pic is a shitty substream pic now instead of a shot from the main stream. Where/How do I change that so I get some nice crisp full-res images?
You need to turn off the sub stream. This is one of the ways a sub stream is a compromise.
While it is not impossible for Blue Iris to begin decoding the main stream each time this snapshot is needed, it would be complex and bug-prone. In cases like yours where a lot (all) of your cameras are configured to save a snapshot at the same interval, then it would cause tremendous load spikes that could easily overwhelm the system and cause instability.