I suspect you need to cast the object to the proper type, but a better solution is to change the dictionary.
If you can't do that, then try
object dict_common_setting ObjGetImgPath;
...
ObjGetImgPath = (dict_common_setting) Global.Dictionary_common_setting["imgpath"];
or
ObjGetImgPath = Global.Dictionary_common_setting["imgpath"] as dict_common_setting;