well, for one, you can flip the pic horizontally, but that's only 2 pics and you'd need 30 version of the same pic or 15 versions of 2 pics, which is already less
obviously you can try cropping, tilting, playing with the colors, hue, saturation, lights, using different filters etc.
i guess it's trial and error until you find a safe setting where they can't detect the pics as copies of each other anymore and it can be that this setting is changing from picture to picture
not sure how advanced their image detection is or what parameters are exactly being used to bust you, but you can also try stripping the metadata - which was already mentioned above - and changing the md5 hash (if you edit the pic even just the slightest, this will change automatically)