Joomla v. 3.7.5
K2 v. 2.7.1

Can't add image either via Media Manger or upload.

No errors given.

Can you try updating to 2.8.0?

I wasn't aware that 2.8.0 was released. I just looked at the download page, and it shows 2.7.1 as the latest version...
www.joomlaworks.net/extensions/free/k2

So.. I went to the update extensions in the site -- and 2.8 as there... I updated, it but it hasn't resolved the issue.

Try going to Joomla Global settings server tab and setting error reporting to development and see if you can get an error
What version of php are you using.
I am using 7. and when i revert back to 5.3 the administrator of my site will not load, I think we are in a transition were some old stuff doesnt work wtih 7. and some new stuff doesnt like 5.

Furthermore check your browser's console tab for any JS issues.

So -- the image I was trying to attach was an 8MP image. like 2 MB. I don't recall there being a limit on the size of the files, but I don't usually attempt images this large, which I think explains what was going on. (still think K2 should be able to handle it, or if it can't there should be an error message reporting the problem, )

Hello,
These limits are set by your hosting provider or your php.ini file.
K2 will not set any PHP limits on its own.