So you want to overwrite the file? or you want to select your wallpaper by selecting an image? original wallpaper is a _manila file that you would need to use cfceditor to extract the png, make changes, re-import/overwrite back into the _manila file and copy to device. It's pretty simple but its semi-permanent (IE you can't change on the fly unless you create several copies to overwrite). Files must be saved with the SAME name to work.
Otherwise, you'd just slide your slider to Settings tab and choose wallpaper to select an image for home page.
Here is a wiki that lets you know the manila files that cover what so you can try editing if that's your flavor:
http://wiki.xda-developers.com/index.php?pagename=Manila_Research