Hi Richard,
Thank you for writing in, no that is not about flexbox. That is just the image being responsive while keeping its aspect ratio.
There are 2 workarounds for that, first is to swap a different resolution image that fits with the column on that certain viewport size using the Hide During Breakpoints option. Second is to apply the image as a background, but as you might already know the background-size: cover; property will cut that image on certain screen size, so if you want to keep the focus on the background-image on the woman you need to set the background-position to center right;
background-size
background-position
Hope it helps,
Cheers!