Good effort, author! Source 4 is a render, and not useable. 5.6. 3D Renders or Illustrations as Source:
Using other people's 3D renders or illustrations (even with permission and no matter in which form they are offered) creates a false impression of your artwork. So 3D renders or illustrations can not be used as sources, unless they are entirely created by yourself. Only acceptable non-illustration brushes and clip art may be used, but you need to mention them as a source too. Don't use an image that has been altered by the source owner nor an entire work created by someone else.
You should be able to replace it easily. All NASA and Hubble images are copyright free.

Ok
Can I use a wallpaper sort like

As CMYK46 mentioned source 4 has been highly manipulated making it a CG render and not acceptable as source. We do allow images with minor adjustments such as those made to NASA and Hubble images but this has been reworked extensively and can't be allowed.

To answer your question, no, wallpaper images cannot be used at all. These are usually altered by their authors and they do not provide information to identify the original source(s). All source images must show ownership/license and/or usage terms for the image(s). You can find information on acceptable sources here: http://www.pxleyes.com/forum/viewtopic.php?f=5&t=5644 and also in the Beginners Guide found here: http://www.pxleyes.com/courses/guide/