Shrink Photo for Mail Signature (2)

Every time I add an image to my signature in the Apple Mail app it changes size on a message depending on the size of the window (same when composing as it is when reading an email sent to myself).

How can I keep it to a specific size that doesn't change (without shrinking the resolution)?

I have followed the instructions in the old/closed thread called "Shrink Photo for Mail Signature" however the instructions are presumably out of date, as there is no "attach" button when creating a signature in the Mail app (see screenshot attached).

Even though I can "Adjust Size" in preview and uncheck "Resample image" to maintain the resolution whilst shrinking the image dimensions, when it is pasted into the signature it continues to change size depending on the size of the window (means if someone maximises a message the image size increases to the width of the screen).

Please assist.

Thanks