It is kind of that easy, but I didn't use a theme structured the way Das+Mini was. I worked with the ones that have a background jpg and a screenshot jpg. I'll make a screencast and show you how I went about doing it. I'll post it here tomorrow.
Here's the text how to for now...
Go to www.gnome-look.org
and download a theme.
I used this one here...it worked for this how to so you may want to use it as well.http://www.gnome-look.org/content/download.php?content=15783&id=1&tan=87618485
Replace the background.jpg with yours it must remain named background.jpg It must be 800x600
Make a duplicate of your image and name it screenshot.jpg
You will need to place a white text box in the duplicate of your image exactly the same place as the one in the original screenshot.jpg
When you have that done replace the original screenshot.jpg with your own.
Then just copy the whole file over into /usr/share/gdm/themes/
Then choose it as a random selection of two, in vasm's gdm-theme-switch. (Just in case something is wrong, your next boot will get you out of it.)
I'll have to get to your other questions about bootsplash tomorrow as well. The ones I can answer anyway.