It is a theme related “issue”, but it isn’t a real issue. Many themes are using bigger images and resize them with CSS, because you will need different size on different screen resolutions.
For example lets say your images are 300px wide, you have a tablet with landscape width of 1000px and portrait is 600px.
If you have 4 columns on landscape, you can use the 250px images. But, if you rotate the device to portrait, your theme will show 2 columns per each row, so you will need 300px wide images.
So when you run the test on the site, and GTMetrix detects that you resized the images with CSS, it will throw a false positive warning. You can’t really do it differently.