Only download image if it has changed

[torbengb]

There's no need to force a download of the image, because it might not have changed. More elegant would be to only save it if it has indeed changed.

• Does wget always download the image but only saves it if it's newer than the local version?
• We can save bandwidth if wgets checks before downloading.

If we can tell that the image has not changed since last download, we can skip the step to set a new wallpaper (gsettings set org.gnome.desktop.background).

• Does wget give us a useful return code?