I'm going to guess that by the term "windowblinds icon's wallpaper" you are referring to the wallpaper shown in the screenshot of any given windowblinds theme being presented.
Those wallpapers are not part of the theme package and are used by the author to compliment their windowblinds. The WB author may or may not be the author of the wallpaper being used.
If you want to know the details of that example wallpaper you could PM the Windowblinds author and ask where they got it.
On RARE occasions they provide that information in the description of the windowblind.
