From the Veracrypt documentation, it looks like they are using a hash of the keyfile contents and the password to derive the header key.
Due to the nature of hashes, a change to any single bit of the input will cause a large number of changes to the calculated hash value. This is true for all hash functions by design.
However, they only use the first 1 MB of data from each keyfile. While a person is extremely unlikely to duplicate your image accurately enough to generate the header key (this is virtually impossible), any file with the same 1st MB of content would work. This won't be an issue with JPGs, and I wouldn't expect it to happen under normal circumstances.
So, in summary, it is virtually impossible for them to unlock the volume without that original file---or at least the first MB of it. Be careful where you keep the keyfile, and securely delete it when moving it.