There is a complaint at the Google Webmaster Help forums from a Flickr user who said he removed an image from Flickr and went through the URL removal process but the image still shows up in the Google search results.
Now this is not uncommon but the response from Google's JohnMu was interesting.
John from Google said:
It does look like Flickr's handling of removed images leaves a bit to be desired in that regard...
Thanks for passing the URL on, squibble. I'll forward it to the team here.
Got that? To me that implies that when Flickr removes an image, it doesn't return the proper status code. So I decided to test it.
I uploaded an image to this URL on Flickr and then deleted it. Flickr returns a page that reads, "The photo you were looking for has been deleted." But if you check the server code, it does not return a proper 404, page not found code, as it should. Instead it returns a code of 200 OK, which means the page is still there. Since that is the case, it confuses Google's spiders and indexer thinking the image was not removed. So speed removals are slowed and it can take longer for the images to be removed from Google.
Forum discussion at Google Webmaster Help.