Featured Image VS Background Theme

I have a little trouble using this theme (free version) because this theme loads autmatically my selected post featured image (FI) as post background image … I made some tests and found it’s affecting my web’s appeareance when FI is small sized and affecting my web performace when FI is large sized.

I need to disable this funcion in all posts and…. use a default background image por all pages and posts without affecting my posts and pages FI.

Any subjection..??

Regards Leon..!!!