Trouble with Featured Images

Hi I’m pretty new to this so please excuse my ignorance. I have moved over from wordpress.com to .org and my site is up and running near enough spot on except for this one thing thats bugging me…
I used to just include pictures in the posts and resize them to 300 x 300 in the options on wordpress.com, however on the self host theme I have to use featured images, which is not a problem however they seem to be not set up right as the different sizes are causing a staggered feel to my homepage … see here … https://www.knightsoftheturntable.co.uk

Is there a way to get around this I’ve tried resizing in the the featured image edit setting but to no joy? is there a plug in that can make them all the same size maybe? any help would be gratefully received! thanks Andy