Hi Marcus,
Not Resize, but Crop them first.
It means focus on (select) area that you want to show and remove what is not needed, and keep the same resolution for all 8 photos.
Use Graphic Tool (even Free have "Crop" feature, GIMP or IrfanView etc) - longer method, but more control over quality.
For example 350x250px - create 8 new cropped photos with new names, and use them in Feature addons, add one by one. You can use .jpg or .webp, to keep smaller file size.
In theory, you can crop using extra CSS, but it would probably too diffult for you.