A site that I regually visit is
http://apod.nasa.govSo just for fun I though to make a downloader to save the picture of the day into /opt/backgrounds so that I may use it as my wallpaper.
#!/bin/sh
# Download Astronomy Picture of the Day
VIEWER="flpicsee"
URL="http://apod.nasa.gov/apod"
PIC="$(wget -O - $URL/astropix.html 2>/dev/null | grep '<a href="image')"
[ "$?" == 0 ] || exit 1
PIC="${PIC#*\"}" && PIC=${PIC%\">} && EXT="${PIC##*.}"
POD="/opt/backgrounds/astropic.$EXT"
wget -q -O "$POD" $URL/$PIC
[ "$?" == 0 ] || exit 1
[ $(which "$VIEWER") ] && "$VIEWER" "$POD" &
Note that I have set VIEWER to flpicsee change to your preferred image viewer. Note too that I overwrite each day's picture. I don't want to fill up my limited space. So if you really want to keep a picture rename it!