a) There were pictures on the wall.
b) There were some pictures on the wall.
They could be different according to context. Imagine going into a slum and being shocked that they had paintings on the wall. There, I wouldn't use 'some' because the number is not the intended meaning, but the very fact that they were there.
a) There weren't pictures on the wall.
b) There weren't any pictures on the wall.
The first sounds a bit incomplete to me.
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