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!