Images resizing

I'm trying to figure out an image sizing issue. I design my courses on a pretty large monitor, but my learners usually take them on smaller monitors. I've attached two screenshots that show how different images can look depending on the size of the monitors. 

My main concern is that when I previewed the course, no matter if on a phone or tablet or computer, I never had an issue with these images sizing incorrectly. You can see the grey boarder at the edges of screenshot 1. Screenshot 2 is what it looks like on my monitor, aka the intended result. 

Is there a way to be sure that this won't happen before I send out to learners? I did my due diligence when reviewing,  but was never able to replicate this error.