1. He was based on mountains and rivers, so he painted a lot of beautiful pictures.

2. He was based on the scenery, so he could paint many pictures containing mountains.

(Are they correct sentences?)

The use of "based on" is causing problems. Here are better ways to phrase these sentences:

1. He had sketched many mountains and rivers, so his paintings were quite beautiful.

2. He frequently sketched scenery, so he could paint mountains well.