There exists site [url removed, login to view] with free API, and there is open-source .NET implementation.
What I require is an WP7 and WP8 app, consisting of single screen with following functionality:
1) Browsing colors and patterns(I though of single pivot with 2 tabs and an wrap panel with small tiles representing each pattern/color(around 99x99) in it.
When user selects tile – new screen showed, filled with this pattern/color.
2) On WP7 at this screen there is appbar button with "save", which saves this pattern/color as 800x480 pic, and message box shows, telling user that pattern is saved and he can navigate to phone settings to set it as lock screen image
3) On WP8 platform this app has lock screen image changing support. At this screen there is same button "save", which asks user whether to use this pattern on lock screen forever, or for now. Navigating to setting this app as current lock screen image providing app should be just like in guidelines.
4) There should be live tile functionality(Wide tile with image), so that when user pins this app to main screen – some random tile should scroll on it.
No more functionality required, so this is rather simple app.