Hello cc...
All of what you are describing is controlled by the client, not by the website. Further, if the website could initiate a file download on its own, it would be a serious security hole.
FTP is a file transfer protocol that works with any client, a GUI one or a text-based one. The interface on the client is controlled by the client. FTP knows nothing of it. So, the FTP server side of things cannot bring up prompts.
Suggestions:
1. Use links with picture thumbnails in them (small images of the full picture). Show the size of the picture next to the thumbnail so the user knows what he is getting into. When he clicks on the image, open a new window to display that image. When he right clicks on the full image and chooses SAVE AS (wording varies), he can download the image.
2. Show the size of a document next to its link also. The user can also right click and choose SAVE AS to download it in this case.
3. Put the image or document file into a WinZIP file. Such a file is never displayed by a browser and is always downloaded whenever it is "opened".
4. Provide instructions on the page about how to download the content. In most/all browsers, you can right click on a link or a picture and choose OPEN or SAVE AS.
Does this help?
-----Steve Jackson
CEO/CSA
Software Corporation (Softcorp)
http://www.softcorp.us/probono
Advanced pro bono tools and utilities free for personal use