Website is very slow to load due to image dimensions

I have been noticing that my website is very slow to load up when I visit it. I have ran a test using gtmetrix and one of the things it is saying is that certain images don’t have the right specified dimensions. I’ve gone through the thumbnail images on the testimonial section and replaced them with ones that are the correct size but the test is still saying they need to be specified. I have read about doing this in css but I’m not sure how to go about it.

I also have a responsive gallery on my front page that changes the size of the thumbnails depending on how small the browser window is set. So I’m not sure how I go about specifying the size of these images if they can be different sizes on different platforms.

My website is https://www.simonfinley.net

Thanks