Hi ToddNeville!
Unfortunately, Rush is only designed for video work and can't export still images. However, this is a great idea and I'd really appreciate you filing a feature request here: Premiere Rush: Hot (1423 ideas) – Adobe video & audio apps
Our engineers check that page regularly (and I send them my favorite links!) to figure out what our next developments of the product will be.
In the meantime, if I were you, I'd navigate to the frame that you're wanting to isolate and take a screenshot of it. If you're on a phone, the screenshot captures the whole screen, but you can crop the screenshot to only show the frame. If you're on a desktop, you can crop the screenshot as you capture it.
Here's how to take a screenshot on a Mac:
How to take a screenshot on your Mac - Apple Support

Here's how to take a screenshot on a PC:
https://support.microsoft.com/en-us/help/13776/windows-10-use-snipping-tool-to-capture-screenshots


This should help get your frame!
Let me know if you have any questions.
-Caroline