Q: How can I change the lockscreen in Fedora 17 using Gnome 3 to match my desktop wallpaper?

A: I was able to find two way to accomplish this.

1) Get your wallpaper and convert it to a png (if it is not already). I used GIMP to open the image, then selected export.  Then I selected png from the file extension drop down and clicked export. I chose the defaults on the export screen.

Then move the new png image to the location that Gnome stores the default wallpaper like this.
(Note: this command should all be on one line)

mv /home/savona/Pictures/hacker_sym.png /usr/share/backgrounds/beefy-miracle/default/wide/beefy-miracle.png

Select yes to overwrite the original.

There are three different defaults, normalish, standard, and wide.  Make sure you select one for the correct aspect of your wallpaper.  I selected wide because my wallpaper was 1920×1080.

2. A more elegant way to do it is to edit the xml file that gnome uses as it’s default theme.  Edit the /usr/share/backgrounds/beefy-miracle/default/beefy-miracle.xml file and change the paths in the code (see below) to point to your image.

        <!-- Wide 16:10 -->
        <size width="1920" height="1200">/usr/share/backgrounds/beefy-miracle/default/wide/beefy-miracle.png</size>
        <!-- Standard 4:3 -->
        <size width="2048" height="1536">/usr/share/backgrounds/beefy-miracle/default/standard/beefy-miracle.png</size>
        <!-- Normalish 5:4 -->
        <size width="1280" height="1024">/usr/share/backgrounds/beefy-miracle/default/normalish/beefy-miracle.png</size>

I think this answers your question.  Thanks for writing!