Help with a recorded simulation in Storyline 360

Hi all, I'm incredible new to Storyline. The version I'm using is 360. I recorded a simulation and had Storyline parse the recording into individual slides. I can't edit the slide directly, so I went to E-learning Heroes to figure out a way to do what I want.

I need to mask out a number on a bunch of slides. I'm using the media library, but I'm having a terrible time identifying the slides I need to edit. The image to the right is too small. Also following the instructions above, I can't close Paint 3D go to the next image open that image and then just paste. I have to recreate the box over and over again.

Is there a quicker way to mask images? Should I just do a bunch of screen shots and forget the recording functionality? I hesitate to do this for fear I'll have to redo all of the navigation. Unless there is a way to globally add navigation (e.g. jump to next slide when user presses Enter)? 

Is there a way to intersperse flat screenshots with the slides from the recording (i.e. recording generated slides and then a slide I create manually).

What is the best workflow for this?