Dropbox did a thing a few years back where they gave you permanent additional storage for uploading photos. A jpeg header followed by gigabytes of zeroes supposedly worked just fine.
This is very easy to catch, though. Abusing comment fields might be more interesting. Use a small valid image, and insert 4MB of data into a comment field. It would be a fun project to implement this as a FUSE file system. I'm not sure whether any of the popular formats support arbitrary-length comments, and whether or not Amazon strips such data (it's image storage, after all, not file storage).
I wouldn't use steganography (it's not normally very space-efficient), just dump the payload data into a PNG's comment field. You can do this with exiftool "-Comment<=/path/to/inputfile" dummy.png and re-extract it with exiftool -b -Comment dummy.png > recovered_file. Of course you'd use libpng directly in a FUSE filesystem, but this shows that it's easy enough. Use a 20kB cat picture and 2 MB payload and you achieve 99% space efficiency.
It's probably inconvenient enough for most data hoarders for Amazon not to care. You can still switch to encoding it in the pixel values in case they do. I'm not familiar enough with PNG to comment on the space-efficiency of that.
This is very easy to catch, though. Abusing comment fields might be more interesting. Use a small valid image, and insert 4MB of data into a comment field. It would be a fun project to implement this as a FUSE file system. I'm not sure whether any of the popular formats support arbitrary-length comments, and whether or not Amazon strips such data (it's image storage, after all, not file storage).