what it is is that those sites (google drive, dropbox, etc.) don't really let you link directly to a file; even when the link appears to point to .zip or .jpg or what have you, what you get is a landing page with some kind of preview. that preview page can't be inlined by the bulletin board software. maybe if you go to the preview page and look at the html you might be able to find the URL for whatever preview jpg pops up on that page, but i suppose there's no guarantee the URL is static.
a site like imgur or astrobin will let you link directly to a jpg if you like. however imgur seems to be somewhat ephemeral and after some period of time the images are removed.
rob