Selecting the size of an image

Hi,
I am using the InfoBox block on our landing page to show certain offerings. With the selected “Medium” size, the images look quite blurry.

Although we have different image sizes registered on our site, the only options I can choose from are “Thumbnail”, “Medium”, “Full”. Is there a way to allow the other registered image sizes available in the media library?

I have tried “Thumbnail”, but that is even more blurry, I also tried “Full” but then some images display in rectangular format instead of a square format.

Any advice?
Best regards,
Andreas

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