In some tasks, image x/y values change by themselves. So the images don't stay "in place".
One example of this is found in task 'Lukujen koonti 1-1000_ver2' (under 'lukimat'). This problem seems to occur mostly with Helena, so I don't know yet if it's somehow operating system/browser/java -version related.
I'll do some testing on my computer and report what I find.