> because you asked and they complied, there's a work contract between said photo-button presser and you.
That's not how contract law works.
> The implicit agreement is that you own the copyright to the photo, and the consideration paid is a word of thanks from you.
Even if there was an otherwise valid contract, with this as an implicit term, you can't transfer copyright ownership from the actual author by implicit agreement: "A transfer of copyright ownership, other than by operation of law, is not valid unless an instrument of conveyance, or a note or memorandum of the transfer, is in writing and signed by the owner of the rights conveyed or such owner’s duly authorized agent." (17 USC Sec. 204)
That's not how contract law works.
> The implicit agreement is that you own the copyright to the photo, and the consideration paid is a word of thanks from you.
Even if there was an otherwise valid contract, with this as an implicit term, you can't transfer copyright ownership from the actual author by implicit agreement: "A transfer of copyright ownership, other than by operation of law, is not valid unless an instrument of conveyance, or a note or memorandum of the transfer, is in writing and signed by the owner of the rights conveyed or such owner’s duly authorized agent." (17 USC Sec. 204)