If the image is yours, and both sites are ones you are working on are yours
or your client's (obviously they must be related in some way for you to want
the same image on both, else I'd just publish theimage to both sites
separately everytime you change the image).
What you would do is
On site 1 put an image tag <img src="
www.site1.com/images/image.jpg">
and on site 2 put an image tag <img src="
www.site1.com/images/image.jpg">
So then you only upload the image to site1 images folder, and site2 will
load that image from site1
This practice is sometimes carried out with webmasters using images on sites
that are not under their control as webmaster, and "take" the image and
place it on their site - called "hotlinking" I think - where the images is
displayed on one web page but is hosted by a completely different ISP or
Host, not related to the other site. it is stealing bandwidth because the
"foreign" image (that not hosted on the site) is being loaded from the other
site, and every time the picture is loaded that's another access to the
other site, which consumes bandwith for that site owner when it is just
loading the image, not actually a visit to that site.
