Adding to your post,
Image resize, crop, visible watermark or caption within image, changing md5 hash would also do the job well.
Thanks for your insight on the matter.
tbh I don't think a MD5 is sufficient. Load an image in paint / gimp / photoshop and save as JPEG 90% quality / 95% quality / 99% quality and you get 3 new MD5 checksums yet the image looks identical to the eye. I would be surprised if the other bots were not doing this before by default before posting / reposting as well as doing a basic bit fiddle just to ensure the MD5 is unique as this is fairly basic knowledge when posting images in SEO / spam bots that any experienced dev should know who has worked in this domain for a while.
So I think ISG are doing something more complex / AI / machine learning / image detection based to detect duplicate images. Only speculation mind, but it would make sense, this is ISG / Facebook we are playng with here who have the resources / skills to do so.
Resize / crop.
I don't know enough about machine learning, but a stretched / compressed image is sort of the same image, not sure if that would be enough on it's own. A cropped image, I think would be better. Resize to 80% of original cropping the 20%, I think would be better. But I have nothing but my assumptions for this.
Watermark
I thiink this would do it, especially if is a big embossed over the top watermark, a little logo in bottom right? well the image is still pretty much the same, so although not a 100% duplicate, perhaps would register as 90%?
Caption within image (am assuming you mean add a bar at the bottom of the image and write text to the image.
Again as with a small corner logo, there will be a high % of a match.
But maybe am overthinking and just a small visual difference is enough, as I said above, been running lots of tests this week with the same base image, but with clear (to the eye) differences in the image, not a problem. So I think as long as the images a bit different to the eye for the time being, it is enough. But I imagine this is an area ISG are looking to improve their detections on and they will I am sure
even uploading a picture taken from my camera got immediately removed.. so its not just the pics
its an overall value of your account and proxies
Account / proxy is definitely playing a part, but that is not the whole picture. I know users with real accounts (were real accounts people used on their phone before botting with them) on their local IP had issues with reposts / duplicate posting (granted these were heavy duplicate posters, posting duplicates all day every day. So maybe ISG are dupe rating each image, this image gets a dupe score of 50%, this one 60% this one 90% this one %70... If you get an overrall score of 60%+ dupe, you get penalised.