wrong flag image size

Google in his testing tool is giving negative points for accessibility.

Due to image size of the flag. I am not sure if it can be changed or but you can look at it.
It expect to have flag image 38×24 pixels instead of 16×12 pixels in certain resolutions.

Maybe it is not an error. But please look at it.

Technical snipped:
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Natural dimensions should be proportional to the display size and the pixel ratio to maximize image clarity. Learn more.

URL

Displayed size

Actual size

Expected size
cs_CZ

…flags/cs_CZ.png(pcpraha.cz)

18 x 12

18 x 12

36 x 24
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The page I need help with: [log in to see the link]