Hi Ollie, Thanks for contacting us! We don’t currently support sharing of images by URL (this is something we will be supporting in the future). At the moment you need to either save to your computer and share from there, or save to Facebook/Flickr/Picasa and share that way. Let us know if you have any further questions!
Sorry, I misunderstood your initial question. This error tends to occur when the image is not publicly available. Ribbet is only able to retrieve images which are both publicly available (don’t require any login etc.) and which are also not blocked by the particular website. Most public URLs will work, for example: https://www.google.com/images/srpr/logo11w.png
If you would like us to investigate the issue further, can you please provide us with an example URL to illustrate the problem you’re running into? Thanks!