Your capture mode is set to the text option. Here's how you can fix it.
When you open Snagit, (not the Editor) look in the lower right where there is the big red capture button. There's a little drop down menu tucked just to the lower left of it. When you click that, you will get to choose from Image, Text or Video.
The default setting is that the Print Screen button just acts as if you clicked the red button. Therefore, when the red button is assigned to Text mode, it'll launch a text capture.
I sure wish I had Snagit handy so I could just show you with a quick picture, but I hope that made sense and is the solution to your problem.
Let me know :)