You forgot to enclose the URL between quotes.
A better solution would be:
Str.Append("<img src='" + url + "' visible='true' runat='server' style='width: 75px; height: 41px' />");
or
Str.AppendFormat("<img src='{0}' visible='true' runat='server' style='width: 75px; height: 41px' />", url);
Kindly note the use of the single quotes inside the double-quotes-delimited-string.