Once upon a time, Google allowed their API to be used by third parties. Roll20 had in-game google image search for this purpose. Eventually Google removed access to this API, but users were allowed to keep the last cached results. All of the "web searches" in the art tab are from this very old cache (at least 5 years, maybe more). If a link has become broken in this time (a web site gone down, or the image no longer hosted) it will still display in the list, but won't be available to be placed. Roll20 has maintained this ancient list for the convenience of when it does work, but they have no ability to edit the results. Use with these limitations in mind.