On FindPenguins, you can upload 10 photos (or 20 photos as a Premium member) into each “Footprint”.
However, as the number of footprints in a trip is not limited, the number of photos you can upload in total is also unlimited, even as a non-Premium member.
The reason for having a limitation is to encourage you to post Footprints in various places: For example: a Footprint on the beach, one at the hotel you're staying at, and one for the national park you visited nearby.