When I upgraded to 2.2.6, My product image gallery is not responsive and resizing on mobile side, and it works well on PC.
Looking at the screenshot i think you are using readymade / custom theme for your project.
To check the responsive on mobile - kindly switch your theme to Magento 2 's default Luma theme and then check
Because looking at the Magento 2.2.6 Vanila version with Luma theme its showing correctly
so you might have issue with your theme !
Hope it helps !