Generate 10000 matrix barcodes
Strings are immutable so whenever you're concatenating them you're effectively creating new instances every time, this can slow down performance a lot, instead use StringBuilder and it's method Append() to build strings.
Avoid duplicate operations per each cycle unless it's necessary, such as Convert.ToInt32(textBox1.Text) convert the value to int before the for loop and save the value in variable, than later use the variable instead of converting the value every time. Another example is comboBox1.SelectedItem.ToString().