Hi willem
You don't say say how you are creating your signature and inserting the jpg in question.
This link -->>
Create signatures with pictures or logos<<-- is the official line which works OK for me.
It assumes that the inserted image is the correct size as it is the one it will use.
There is also a link within that page which advises on how to -->>
Insert a signature in a message<<--
I apologise if this advice is in fact how you do create your signature, but there are some folks who insert manually text and an image in every email they send as part of the body of that email.
This of course does show the intended wording and/or image, but it's not the method used for a more automated 'signature'.
Hope this helps.