Ann's clothes are covered in paint.
She has been painting the ceiling.
Has been painting is the present perfect continuous tense.
We are interested in the action. It does not matter whether something has been finished or not. In the example, the action has not been finished.
The ceiling was white. Now it's blue.
She has painted the ceiling.
Has painted is the present perfect simple tense.
This time, the important thing is that something has been finished. We are interested in the result of the action, not in the action itself.
Here are some pairs of examples:
Tom's hands are very dirty. He has been fixing the car.
You've been smoking too much lately. You should smoke less.
The car is working again now. Tom has fixed it.
Somebody has smoked all my cigarettes. The packet is empty.
We use the continuous form to say how long something has been happening:
Ann has been writing letters all day.
How long have you been reading that book?
Jim has been playing tennis since 2:00.
We use the simple form to say how much wc have done, how many things we have done, or how many times we have done something:
Ann has written ten letters today.
How many pages of that book have you read?
Jim has played tennis three times this week.