Okay,
Let's try a couple of things:
1 - Use the following to look in the registry: "Start > Run > type: regedit > press 'Enter' > in the left pane, expand the folders until you reach the following address: "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies".
Is there a "System" folder (key) listed inside the above folder (key)?
If so, highlight that "System" folder (key) by clicking on it. Is there a "Data" value in the right-hand pane that says: "C:\WINDOWS\Web\Wallpaper\background.html"?
If there is a value as listed above, right-click the "Name" to the left of that value (still in the right-hand pane of the registry editor) > select "Delete" > confirm deletion > reboot machine > attempt to change the wallpaper.
2 - If there was no "System" folder (key) found in the specific folder (key) above, you can go here http://www.kellys-korner-xp.com/xp_tweaks.htm and download the "Allow Wallpaper/Background Changes" from line #142 in the right column (note - there are actually 2 downloads via that item in the right column, so make sure not to download the "Disable" portion of that line item).
After downloading that ".reg" file, you would simply double-click the file and say "Yes" to writing to the registry. This file sets a number of keys in the registry that have to do with wallpaper "policies" (most notably the "active desktop" keys) back to their default settings.
Another way to do this is to change the downloaded ".reg" file to a ".txt" extension and open the file in Notepad, which will allow you to see the keys and values being set. If you like to work in the registry, and just want to practice (or change manually) you can go to each key and see if the values are already set to the defaults indicated in the file.
If the above does not work, I suspect that whatever occurred during your absence from the computer (and overwrote the original "Luna" theme in Theme Manager), may be the culprit. If this occurred while a theme was applied, then something may have been holding on during the change and is either still holding on, or has been deleted and the Display Properties file responsible for changing the background image has been affected in some way.
Final note - the solid background color change is common using WindowBlinds skins due to the color scheme used by the author of the skin.