Look for each cycle's Wallpaper Contest on http://discourse.ubuntu.com. Save up your best images for the next contest, and submit then.
Here's an extract from the 21.10 Contest announcement listing the criteria and best practices for wallpaper submissions:
There are a few rules though. Please make sure you are happy to submit your work under these rules before you post it. Posting your images here indicates acceptance of the rules.
Ownership
The images you upload must be yours. If you see an image which you know to be misrepresented please flag it and we will take care of it.
Size
You don’t have to upload the full size image to discourse. In fact, if might be easier if you don’t since this thread will become very slow to load. We will need an image of ideally 3840x2160 px for the final submission, but we can get that from you later on if your image is selected. For now, limit your images a bit to try and keep the file size down.
Quality
As mentioned above, the final image size needs to be 3840x2160 px. Images much smaller than this won’t scale up well. Equally images with a lot of compression artifacts won’t look good. Your image won’t be accepted if the quality is low.
Watermarking
All watermarks, names, logos, etc must be removed.
Licence
Your image(s) must licenced under the CC BY-SA 4.06 or CC BY 4.03 licence. If you do not specify a licence in your post then we will assume that you are are licensing the image under the CC BY-SA 4.0. By entering the competition you are agreeing to these licence terms.
