It is possible that the CSS you are trying is simply not supported by your email client. Email clients typically support a significantly reduced subset of HTML and only minimal CSS. When composing emails, HTML is often the right reason ... HTML table layout, etc., to get the most support.
EDIT: IMO trying to crop an image using HTML / CSS will not be possible if you want this to work across all email clients. Relevant properties that could help achieve this: background image, height, overflow, clip, etc. Just not supported all over the board. So, although this will work for some, it will not be for others.
An alternative would be to reduce the width so that the height is always 100 pixels. You will need to calculate the width of the server side in order to maintain an appropriate aspect ratio.
However, I think the best solution would be to create a (more) square 100px image on the server side with PHP?
MrWhite
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