<img src='' />
While there is no valid way to omit an image's source, there are sources which won't cause server hits. I recently had a similar issue with
iframes and determined
//:0 to be the best option. No, really!
// (omitting the protocol) causes the protocol of the current page to be used, preventing "insecure content" warnings in HTTPS pages. Skipping the host name isn't necessary, but makes it shorter. Finally, a port of
:0 ensures that a server request can't be made (it isn't a valid port, according to the spec).
This is the only URL which I found caused no server hits or error messages in any browser. The usual choice —
This was tested in Chrome, Safari 5, FF 3.6, and IE 6/7/8, but I would expect it to work in any browser, as it should be the network layer which kills any attempted request.