A question about 了 and a verb 挂
1.墙上挂一幅画


Just a neutral description and the meaning is context-dependent


  2.墙上在挂一幅画


Your translation is correct


  3.墙上挂着一幅画
  
  4.墙上挂了一幅画


I don't see any difference.Both means "A picture is hanging on the wall"


  5.墙上挂着了一幅画


This is gramatically incorrect and doesn't make any sense