There are different perspectives for visualizing a stasis painting.
In Day of the Doctor, we saw Gallifreyian saying,
We'd lost in another dimension.
It means that when you create a stasis painting, one moment is frozen in time from our Spacetime perspective, but the contents are actually pushed to another dimension.
Now, the nature of the another dimension: It can have no time or it can have time. Just because it's frozen for us, it doesn't mean the time doesn't exist for that dimension. It simply means that time of that dimension is simply out of sync with ours (say their one second = our millions years; in theory, it should be our infinite years if it's really frozen).
In Time of the Doctor, we saw that Gallifreyians weren't actually frozen in another dimension.
Final Answer: The contents of stasis painting exist in another dimension with its time out of sync with ours. When you see Doctors moving that's from other dimensional perspective. As for how Daleks would fly out of that dimension to ours: We don't know the exact nature of stasis painting. Maybe, it puts a higher dimensional variable constraints on contents which Doctors broke with Sonic Screwdriver.
P.S. The reasoning hasn't been mentioned in the show. It's my theory and this is the best I can think of.