Would you tell me if I am right?
The only difference between the following is the fact that the period of time as to first one is longer than the second one-- emphasis.
"I still do not have the picture"
"I do not have the picture yet"
Both the adverbs mean the same that when you speak that sentence the matter in concern is not done. It is pending, in other words.
But then, there is a slight (but indeed interesting) difference! My source for this answer is British Council's Learn English:
‘Yet’ is used to talk about something which is expected to happen. It means ‘at any time up to now’. It is used in questions and negatives.
yet usually comes in the end of the sentence.
On the other hand,
‘Still’ is used to talk about something that hasn’t finished – especially when we expected it to finish earlier.
The example follows:
You promised to give me that report yesterday and you still haven’t finished it.
That said, both actually mean the delay or things that have not happened till now. I don't think there's a 'time constraint' in this the way you think.