What do you get with a free account?
A free account has 1 GB of storage to try out photos and video. Viewing shared albums and posting text updates stays free forever.
With 1 GB you upload a few hundred photos or several short videos: enough to build an album, fill your timeline and see how sharing with family works, without paying anything. There are no ads in return and your data is not sold; the free account is an introduction, not a business model.
If someone invites you to an album, a free account is enough to view everything and react. Posting text updates on your own timeline also stays free forever.
When your archive grows, you pick a roomier plan later, possibly shared with your family. If a large photo doesn't quite fit the free space, you can shrink it first with the free resize an image tool.