How can I get a random image on my lockscreen? Could you recommend me a simple solution to set a random image for my lockscreen? 

I have the stock Samsung launcher, stock ICS rom and I don't want to install any custom launcher. I'm just looking for an app to switch the image every few hours.
You can use a live wallpaper that downloads new images periodicaly.
This one has a lot of built in image sites (like national geographic, nasa, ...):

https://play.google.com/store/apps/details?id=com.dngames.picofday

It looks like a normal wallpaper and is battery friendly.
The only problem I see is that most of the built in sources update once per day not every few hours, but you can define your own source of images.

You can also try searching for other similar solutions. I think the livewallpaper way is the best beacuse it has a lot less issues with lockscreens (you just choose the live wallpaper as the lockscreen wallpaper and it should leave the normal background intact.