The data you store in the Session already has the data:image/gif prefix, however you add it again in your view, so just miss that step out
@{
var img = (Session["UserImage"]);
}
<img src="@img" width="60" height="60" class="img-profile rounded-circle" />
Also you don't seem to be converting the byte array to base64, if dataitem.UserImage is a byte array then use
string imageDataURL = string.Format("data:image/png;base64,{0}", System.Convert.ToBase64String(dataitem.UserImage));