asp.net gif as progress bar

hello everyone :-)

i have a button that runat="server" and i want to show a progress GIF while the send process is running.
i tried to do that with img="bla.gif" visible=false and true. it doesn't working.
then i tried to do that with 2 Threads that running together - doesn't working at all.

do you have any other suggestion for this case? pay attention that the button must run on server because it is an email sending.

thank you!!!
Ido.