The point for Supplemental Wallpapers is not simple to have a package with fine wallpapers as to bring people to contributions. To collect simple photographs that are public domain or with an other proper license is not the way you will get contributors just you will get a lot of submissions. As ubuntu design team announced the winners for there next version now, I could go easy through the submissions and pick nice ones and submit them by myself but I dont do that.
I prefer the way to write the submitters and show them that there are other distributions, where they can submit there work and leave the decision to do it to them. This way we create contributors! Example just take a look at Cleide Izabel, you will find her also always in the Ubuntu contest but she also submitted for F18, F19 and now F20 and won, I would call her definitely an contributor now.
To collect such work was done in earlier Supplemental Wallpaper Contest, but did it move us forward? Why didnt we had one for F17 someone lost the pleasure to collect others work. openSUSE does it that way but look at the results. Personally I prefer quality not quantity, if we have 30 submissions in high quality its enough to have an voting and get an nice package.