Use whatever image hosting service you like and post the link to /r/woodworking. The key here is it needs to be apparent that you've built the entry, and you follow the photo requirements listed in the contest rules. Submissions of finished photos only will not be accepted for entry. <-- I can't emphasize this enough, you absolutely need more than 2-3 photos for a valid submission. I will no longer accept entries that have albums that go from rough stock to 90% complete in one jump.
The shelf in the first picture is made of red oak plywood. You can choose the wood type, color and design as you like for your project. In case if you need more help understanding this project, you can refer the source link below. It discusses various items used, steps and tips and personal experience of the author who personally built a Zigzag shelf.