Make images of different aspect ratio the same size

I don't know how to make these images the same size so that they have a same size container, I have a width of 30% on my .game span that contains the image but do I need to add a height ?? If so I don't know what units to put, every solution keep telling me to put pixels but I feel like it's a big no no.
https://codepen.io/Antony-Collin/pen/WNYejKZ
image.png
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