Is there a way to scale a photo to fit the screen without the ease of a button that I can
do on my own?
If your screen is 1024x768 do:
cd to dir_where_picture_is, then
convert -resize 1024x768 desktop_pic.png desktop_pic.png
move it to /usr/share/backgrounds if you want to but you don't need.
Just browse to it.
Edik