I'm not exactly sure what you're looking for (random wallpapers or specific wallpapers at different times of the day).
Flikkie will switch it every 6 hours if you want (so you could do Morning, Afternoon, Evening, Night) but you'd have to kick it off at (for example), noon to have it switching at noon, 6pm, midnight, 6am....
I use Setting Profiles (as someone else mentioned). Even though I use it for other things (like location), you could use it this way easy enough. Basically you would set up a profile and assign it a wallpaper, then tell it to activate that profile during a certain time.
As an example, I do have a "Night" profile that is automatically used from 11PM to 6AM. While I use it to silence and turn off everything but the ringer during those hours, I also have it set to display a "night-time" wallpaper (a pic of city at night).
I have several other profiles with their own wallpapers as well, such as "In a Meeting" (pic of guy yawning) and "In the Car" (pic of a road stretching off into the distance)
Having different wallpapers for each profile allows me to easily identify which profile I am in, since I have several for different situations and switch back and forth throughout the day.
You'd just need to set up as many profiles as you want, assign a wallpaper and a time to have each profile active and you'd be done. Obviously this could be a pain if you want it switch every 15 minutes since you'd need a ton of profiles, but if you'd want one for "Morning, Afternoon, Evening, and Night" it wouldn't be too bad.
Although $4 for the app is a bit much just to switch your wallpaper.
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