For AutoCompleteExtender You are using UserId as Image Path as it added in Searchusernames list you need to pass the ImageName column value to display the image also check the proper src path in usernames_Populated after modifying the searchEmployee Webmethod.
First change the searchEmployee with below line of code.
Searchusernames.Add(AjaxControlToolkit.AutoCompleteExtender.CreateAutoCompleteItem(string.Format("{0} {1}", sdr["Name"], sdr["UserName"]), sdr["ImageName"].ToString()));
Then check the usernames_Populated in which you are setting innerHTML for Image Tag with src path assigned for images.
Also if you are talking value of ImagePath column then remove .jpg from innerHTML once you change in searchEmployee method.
eg. Check the below code and modify according to your method
div.innerHTML = "<img style = 'height:50px;width:50px' src = 'photos/"
+ employees[i]._value + "' /><br />"