The definition of "published" is when your images are made available for sale (send a file to the client or put it on a website to make t-shirt sales, etc.). You then have 3 months to get it registered to obtain full legal remedies protection for your art. If you wait longer than 90 days, you have protection from when you submitted the Copyright form, not from the time you made it available for sale.
You can register a bunch of images at once if they are unpublished, but in court the judge may interpret the collection as one work and feel a partial infringement of one part of the collection is not worthy of heavy fines...
You can register published works together (all the images in a book or article for example), or any collection of images all published on the same day. Again you may run some risk that the judge will find an infringement of one of these pieces in the collection to be less vile than a single image registered separately.
My opinion is that small groups of images register as unpublished is fine, and registering a modest collection of work published on the same day is also fine, otherwise images published separately should be registered separately- but I am not a lawyer...I just talk to them occasionally. Especially if the reuse likelihood is small, you may forgo registration altogether or register an unpublished group as a small nod to protecting your art.
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