Create a simple stamp in Adobe Reader nothing seems to work

I have made a stamp with a basic signature from a scan of the paper/ink signature saved as PNG, resized to the correct image proportions, and imported into Adobe Acrobat and it works fine.

I now needed to add this to just Adobe reader on other pcs to just stamp on received invoices, reader has no picture option only pdf but the correctly sized image is huge as a stamp and although can be resized its a total pain each time.

There seems to be zero tools in reader to do anything with the stamp to make it the correct size and all of the guides are only for Acrobat which doesn't need them.

How can I get this signature onto a pdf and then used at a sensible size in Reader? it shouldnt be this difficult and should just save the size you make it.