When I upload images (from FurAffinity, more specifically) by right-clicking on the Download link, clicking "Copy link" and then pasting the link into the upload box, I'll get an "Image Not Found" icon.
This is worked around by going back on FurAffinity and clicking "Download" on the image (open it in a new tab if you want) and it will then show the image in the upload box.
If it helps I'm using Chrome in Incognito Mode with Adblock Plus enabled (ABP doesn't block e6's ads, and I wouldn't want to block e6's ads anyway because I want to support this site!)
Updated