you can use the visual arts form ( form va )
on the copyright.gov website.
i have attached a pdf of it, hopefully it makes sense.
it is either for single registration or a "gang" registration of a whole bunch of your images.
it is also OK to make copies of the images, and mail them to yourself ...
basically submit them to yourself, make sure the envelope is POSTMARKED
and you don't open it when you receive it back in the mail.
(put it in a safe place, as you would your certificate of registration)
it is the cost of a stamp, instead of the 35-40$ .. and can still be used
as "proof" if you get into a jam.
i don't remember what the backlog is at the copyright office, it might take a few (8?) weeks
to get your certificate, they are busy ..