Sure, and in this case, the protocol-level semantic is the size of the OP_RETURN, regardless of whether it is a jpeg or some other data. The point is, it has no effect on whether Bitcoin is permissionless so long as it is not enforced arbitrarily, such that some transactions with larger OP_Returns are permitted, while others are not. Therefore your argument that it violates the ethical principles of Bitcoin is invalid. All users and all types of data within the OP_Return are treated the same by Knots. It's not looking to see whether that data is a jpeg, just whether it exceeds the size limit.