The green gallery tab is the fourth tab down in your aquapedia and is a little camera icon. It lets you see all of the images that you have taken so far with your Splash-Cam. There is no limit to how many pictures you can take, so fill up the aquapedia with as many as you like.
If you tap on a picture and select "customize", you can put stickers on your pictures, but please see the Splashcam FAQ section for more info on this.
If your game is uninstalled or your device reformatted - any images that have not been saved elsewhere or shared will be lost.