For the last two weeks, It's been happening every day. = It happened/It was happening (yesterday, the day before and every day until two weeks ago).

[Marika33]

If I say, "For the last two weeks, I've been studying English for two hours every day, from 7 to 9 pm.", it means that I studied English yesterday from 7 to 9 pm, the day before and every day until a fortnight ago.

But if I say, "For the last two weeks, I've been painting this picture for two hours every day, from 7 to 9pm.", which does it mean?
1. Yesterday (and the day before and every day until two weeks ago,) I was painting this picture for two hours, from 7 to 9 pm.
2. Yesterday (and the day before and every day until two weeks ago,) I painted this picture for two hours, from 7 to 9 pm.

 

[Piscean]

If I say, "For the last two weeks, I've been studying English for two hours every day, from 7 to 9 pm.", it means that I studied English yesterday from 7 to 9 pm, the day before and every day until a fortnight ago.

You could also say I was studying English yesterday from 7 to 9 pm, the day before and every day until a fortnight ago.

But if I say, "For the last two weeks, I've been painting this picture for two hours every day, from 7 to 9pm.", which does it mean?
1. Yesterday (and the day before and every day until two weeks ago,) I was painting this picture for two hours, from 7 to 9 pm.
2. Yesterday (and the day before and every day until two weeks ago,) I painted this picture for two hours, from 7 to 9 pm.

Both versions are possible.

In each case, the version with the progressive aspect places more emphasis on the duration of the activity.

 

[Marika33]

If I say, "For the last two weeks, I've been painting this picture for two hours every day, from 7 to 9pm.", which does it mean?
1. Yesterday (and the day before and every day until two weeks ago,) I was painting this picture for two hours, from 7 till 9 pm.
2. Yesterday (and the day before and every day until two weeks ago,) I painted this picture for two hours, from 7 till 9 pm.

Both versions are possible.

In each case, the version with the progressive aspect places more emphasis on the duration of the activity.

Thank you very much! This helped!
I just thought that the version with the past simple might not be correct due to the fact that it might indicate (as emsr2d2 said here) that the picture got completed each day (which I was not intending to convey).

For myself: I can choose either of these two:

• I've been painting this picture for the last two weeks for two hours every day, from 7 to 9 pm. I did it yesterday, I did it the day before yesterday, I did it three days ago and so on.
  (did = painted the picture from 7 to 9 pm.)
  
  
• I've been painting this picture for the last two weeks for two hours every day, from 7 to 9 pm. I was doing it yesterday, I was doing it the day before yesterday, I was doing it three days ago and so on.
  (was doing = was painting the picture from 7 to 9 pm.)

 

[Tarheel]

@Marika33
Frankly, I wouldn't say that all. If I was painting a picture I might say that I've been working on it every day for two hours a day.

I think you might be confusing yourself.

 

[Piscean]

For myself: I can choose either of these two:

• I've been painting this picture for the last two weeks for two hours every day, from 7 to 9 pm. I did it yesterday, I did it the day before yesterday, I did it three days ago and so on.
  
  
• I've been painting this picture for the last two weeks for two hours every day, from 7 to 9 pm. I was doing it yesterday, I was doing it the day before yesterday, I was doing it three days ago and so on.

You can.

 

[Tarheel]

In theory you can use anything that's grammatical.

 

[Marika33]

Frankly, I wouldn't say that all. If I was painting a picture I might say that I've been working on it every day for two hours a day.

I think you might be confusing yourself.

I don't think I'm confusing myself. I know that I can say "I've been working on (painting) this picture for the last two weeks for two hours every day, from 7 to 9 pm" instead, but I have heard people just use the main verb "paint" without this construction "work on painting".

 

[Tarheel]

No, that's not it. It's not "working on it (the painting).