Problem Opening Stills in External Editor
I created a freeze frame still from the current video I am editing
If you used the | Shift n | command to make the freeze frame, it isn't recognized as a still image in any program other then FCP. To make an actual still image file that will be recognized and usable in other programs, you have to export as a still image. Set the Timeline playhead on the frame you want, then go to File->Export->Using QuickTime Conversion. In the FORMAT menu, choose "Still Image" and in the OPTIONS menu, choose the file type (JPG, TIFF, PNG, etc). Import the resulting image file into your project and the "Open in External Editor" command will work.