You can install Variety which will do what you require.
In Menu, open Terminal, then type the following, pressing Enter after each command.
sudo add-apt-repository ppa:peterlevi/ppa
Give your user password and press Enter again.
sudo apt-get update
sudo apt-get install variety
Once Variety is installed you can uncheck/untick the boxes for various wallpaper sources and press Add.
Then select your own pictures for use as continually changing screensavers.
You can also vary the time interval between changes.