I am not sure if this works on every version of Windows 7 but on my Windows 7 Ultimate 64-bit it does. I also did not test this out when changing themes. If I manually changed the background picture or select a folder of pictures to slideshow through then the registry value that I query below seems to always be updated with the path to the current image.
There are several ways to make this better (like automatically opening Windows Explorer to the folder the picture is in or launching the picture directly) but this could get you started.
I created a batch file on my Desktop named path2pic.bat and had the following code in there:
echo The path to the current Desktop background image is:
REG QUERY "HKCU\Software\Microsoft\Internet Explorer\Desktop\General" /v WallpaperSource
Then I created a shortcut to the batch file, assigned a Shortcut Key like Ctrl+Shift+P to it, and any time I want to know the path to the image I press the Shortkut Keys and the batch file shows me the path.