An original, an edition or a book?
The same image can be encountered as a single painting, a print or a page. Decide how you want to live with the work: on a particular wall, as one print in a collection, or in a sequence you can turn by hand.
An original
Ask about the specific work. Its title, artist and medium are more useful than a general description if you know them. Ask for the artwork dimensions and the overall framed dimensions separately, and tell us where you imagine it will hang.
An edition
Identify the image or release. Ask about the sheet and image sizes, the edition information, and the presentation of the available copy. An image online does not establish that a particular print is still available.
A book or publication
For a personal copy, a title or subject is a good start. For a shop or reading room, include the quantity, destination and any opening date. The house can confirm the title and available distribution arrangements by correspondence.