Diplaying Animated Gif

I'm building an upload page and I'm using javascipt to show/hide an
animated gif when they click the upload button but when it is displayed
the animation stops during the upload process? Is this because of the
cpu load or any ideas on what could be causing this? I'm using PHP to
upload the file. Here is my code to show/hide the gif.

function UploadImage( f ){
if(f.userfile.value.length < 1)
{
f.bUpload.disabled = false;
f.bCancel.disabled = false;
f.bUpload.value = 'Upload'
alert("Please select a file to upload!");
f.userfile.focus();
return false;
}
else {
f.bUpload.disabled = true;
f.bCancel.disabled = true;
f.bUpload.value = 'Uploading....Please Wait'
return true;
}
}