React, Mobile-first approach, Flexbox

What are you most proud of, and what would you do differently next time?

Project built using React framework and mobile first approach. Feedbacks are always widely welcome and will be taken into consideration.

What specific areas of your project would you like help with?

I am looking for a specific feedback on my way of handling layout at different screen sizes.

1. Is it common way of handling responsive design, and conditionally rendering components as I did. Is there a better way, and which is it, using best practices?
2. I think there might be a small bug on ImageGallery on first render of the component, since it looks like large image is being shown right away, and then there is that flashing effect, which should only take place upon changing the image, it also reffers to Lightbox component since ImageGallery is a part of the Lightbox as well. I am wondering how do I remove this 'buggy' behaviour?