Sorry it took me so long to respond.
The issue has pretty much resolved, but I think that is because I cleared out my "gallery". I'm also now using a picture saved in my gallery (from Zedge).
When the wallpaper/theme was changing on its own, I was using a live wallpaper (a "GO" wallpaper, Lord of the Rings themed and Blue Planet). I'm not 100% certain, but I do believe it changed itself pretty often while I was charging. I use both a wall charger and a car charger, never have used a USB cable with the computer.
I'm unaware of a "task killer" on my phone. I do have a "task manager", but I believe that was on the phone when I got it.
I might like to go back to the Blue Planet theme, or one of the LOTR themes if I am sure it will stay put lol.