So these are the 18een Gnome Wallpapers
You will find them in Gnome Git Repo: http://git.gnome.org/browse/gnome-backgrounds/tree/backgrounds (click on plain). Everything is on high resolution, but I didn’t find the source files.
Gnome Photos is one of these applications that are suitable for new contributors to hack them. It is written in C and is still on early stages so there isn’t much code to read.
If you want to contribute:
Photos, like Documents, Music and Videos, is one of the core GNOME applications meant for find and reminding the user about her content. The internal architecture Photos is based on Documents — the document manager application for GNOME, because they share similar UI/UX patterns and objectives.
Points to remember while hacking on Photos:
+ Share the same set of widgets as Documents, or the other core GNOME applications, as much as possible.
+ Monitor changes in the Documents code base and clone them when relevant.
+ Follow the GNU coding style. To accomodate longer class and method names due to namespace prefixes, line lengths up to 120 characters are allowed.
happy hacking ;)