^^^
Invoice showing terms was sent in email with links to pictures, they then used the pictures, this means they accepted the contract therefore there is an agreement to pay.
It's not as clear cut as this, for either party.
As you say, OP never signalled acceptance of the papers terms before sending images in.
Similarly, the paper never signalled acceptance of the invoice before using the images.
Normally I would agree, that usage of the images constituted an agreement, but the fact that the OP changed the proposed deal with no negotiation means that to me, the deal is questionable.
If I was the paper and proposed the terms to someone, and then that person sent the images, with no further negotiation, other than including an invoice (Which could be easilly overlooked given that it would have been an attachment rather than in the email body), I might have used the images also.
As for agreeing terms etc yes perhaps it should have been made absolutely clear but like i said before I've sent photos with the invoice to other papers and there's been no problem with them paying. I was just going by personal experience.

