How to improve Gtranslate Plugin rating

Hi. How to improve the result from the assessment at https://web.dev/measure. The Gtranslate plugin is lowering my rating.
When entering https://sperkydita.cz/ in web.dev/measure. Message. Assessment only 83 Best Practices
Serves images with low resolution
Image natural dimensions should be proportional to the display size and the pixel ratio to maximize image clarity.

cs
<img src="//sperkydita.cz/wp-content/plugins/gtranslate/flags/16/cs.png" height="16" width="16" alt="cs" data-lazy-src="//sperkydita.cz/wp-content/plugins/gtranslate/flags/16/cs.png" data-ll-status="loaded" class="entered lazyloaded">
…16/cs.png(sperkydita.cz)
16 x 16
16 x 16
32 x 32

How can I fix it on the website https://sperkydita.cz/. Thank you for the advice.
Regards George

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