How to avoid a person clicking submit more then once

Hi All.

I have an excel processing webpage that sometimes does 2000+ lines of code. This can take 5 seconds max. In the mean time a user might think that nothing is happening and will procede to click submit again. I do not want this to happen...

So I thought it would be nice to set the submit button to invisible and set an image to visible. However the webpage just hangs there and my image never displays. How do I go about doing this?

Here is the code I am using right now:
protected void ReadExcelButton_Click(object sender, EventArgs e)
< ...