Static use to pay attention to the place

protected static string headimg = String. Empty;

  Use static here, if you write like this

Object himg = DBUtility.DbHelperSQL.GetSingle (HSTR);

       if (himg! = null) {headimg = "http://www.16njl.com" + convert.tostring (himg); }
Assign a value to Headimg, there is assignment, no data is not processed headimg  
This will cause an error, when you do not re-assign the value of the assignment headimg, the value of headimg will be saved, even if you re-login, the value as long as it has not been re-assigned to exist (there may be a time limit, but a long time), the next account does not change headimg Value, an error occurs with the headimg of the previous account.

   protected static string headimg = string. Empty;        protected void page_load (Object sender,  eventargs e)         {
} 
 This method of static is not changed because of the account headimg== String.  Empty; The value of the headimg does not change unless the value is re-assigned within the function. 
 
 So be sure to write this way: 

  object himg = dbutility.dbhelpersql.getsingle (HSTR);                 if  (himg != null)                  {                     headimg =   "http://www.16njl.com"  + convert.tostring (himg);                 }                 else                 {                      headimg =  ""; Remember to re-assign the value         &Nbsp;       } 

or remove the static.

Static use to pay attention to the place