Copy the link from the webpage of your listing and the picture shows up automatically once you post it.
I'm doing digital downloads now. I can scan up a 1200 dpi resolution / 400MB bitmap file if need be, but otherwise, I'm sending things out in the 10,000 x 7200 range (19MB) JPEG files.
For point of reference, newer digital cameras typically take 12-14MB pictures, so my files are larger and you can zoom in to the point where the lines from the original printing plates are visible in shaded sections.
Here is an example:
http://www.etsy.com/listing/93013811/digital-download-1915-europe-mapI had Iceland about 2-3" high on my computer screen from this file yesterday and it looked good.
I'll see where this goes, but so far I've had good response.
Good luck to everyone with your sales!
Val