The two wedding codes that get used: one on the invitation to the wedding website or RSVP form, and one on the day, on every table, to a shared album where guests drop their photos (a shared iCloud or Google Photos album, or a service built for it). A third for the playlist is optional and fun.
Invitations are read close up, so a 2.5 cm code is enough; table signs are read from a seat, 3 to 4 cm. Printers love SVG: send them the file and the code stays crisp on textured card. Soft shapes (leaf, rounded, dots) and a muted colour suit stationery; keep the dots dark enough and leave the margin.
The album link is the one most likely to change (services get replaced, links get regenerated). If the stationery is printed months ahead, use a lifetime editable code and repoint it the week before.
Questions
Which photo-sharing service should the code point to?
Any shared album link works. iCloud Shared Albums and Google Photos shared albums are free; dedicated wedding services add a slideshow for the day. Pick one that works for guests without an account.
Can I put our photo inside the code?
Yes, that is the Picture tab. A portrait against a plain background works best. Print it at 5 cm or more.