It's currently 4MB, Annemie, though we appreciate it when people can reduce the size a bit - but we know not everyone has software to do that. We want to make sure people can load pictures that are good quality at full screen - but a lot of cameras take pictures that are much higher than that.
We've also learned that having a long name for a picture can cause an error message about picture size, even though the actual picture is not that big - I haven't done any testing, but know that up to 30 characters is fine.
I hope this helps, and if you have other questions, please let us know.