I'm having trouble with the way an image appears when I use the "Insert / Type / Paste Custom Text" action in BetterTouchTool. My BTT shortcut is a keyboard sequence that pastes my work email template, but the company logo image in the email signature always appears larger than it does as configured in the BTT editor.
I think it will always paste the original image size, if you need to format the image you'd probably need to use some other paste option or resize the image file itself.
Providing your signature as HTML should work.
Unfortunately I seem to have broken the paste format options recently. This should work again with v4.069 alpha