How do I create a banner for email signature which includes a hyperlink on Outlook

Graphics tend to appear bigger in Outlook emails than expected to me, but I just did a quick Google and found 96dpi as the default.  So I tried that with a 100mm by 50mm @ 96dpi test graphic, and it came out about right.

So I tried another test graphic the same size but with 72dpi, so this one had less pixels.  And guess what?  It came into an Outlook email exactly the same size as the first one, so Outlook uses the DPI and sizing of the graphic. 

Just to make sure, I tried again at 300dpi, and again it hounoured the image size and exactly matched the first two.  Only this time, the 3 pixel stroke I'd put round the border to mark the boundry, was way thinner.

So there you are.  I'd say 96dpi was a good compromise in these days of HiDef screens.  I'd probably aim at an 8 inch wide page.  96DPI is also going to give you a good indication of how elements in the graphic will appear in the email/hyperlink.

[EDIT]  BTW  Just  a quick thankyou for making me work that out, because it's something I could have made use of many times, but nevre bothered to work out.