Easiest way is to just add IMG tag.
cell5.Text = string.Format("<img src='{0}' />", blaa == "1"? "Male.png" : "Female.png");
string.Format
will substitute {0} with the second parameter - which will select either male or female based on value of "blaa".
If you use the code as is - you should place the images into the same directory as your ASPX file.