[Casiopea]

'He was stood there saying nothing.' - (Or is the correct form - 'he was standing there saying nothing.')

He stood there, saying nothing. (Simple Past)
He was standing there, saying nothing. (Past Continuous)

'I like laying on the grass and looking at the clouds.' - (Or is the correct form - 'I like lying on the grass and looking at the clouds.')

I like lying on the grass.
=> 'lay' is an action, 'lie' is a position.

'It's the most beautiful painting I have ever seen.' (Or is the correct form 'It's the most beautiful painting I've ever seen.' Or 'That is the most beautiful painting I have ever seen.'

All are fine.

All the best,