Remove Masonry
First of all: Your sketches are awesome! Maybe i'll use in my LARP project, if you like to check, i'ts here, but it's not important: https://www.figma.com/proto/a2dqo8xOwDXROnOAhSNWyBz1/Imperium-LARP?node-id=304%3A205. May I use in-game not at website...
Now, Masonry plugin... You can remove these plugin to prevent these:
Because, masonry set hight in the row.masonry-container and the images load after plugins run, causing this.
Now, the bootstrap has the display:flex, and you don't need use this plugin anymore, check:
Check this out using only css: https://codepen.io/ronaldo-hoch/pen/eYdZbgd