creat a fading effect?

I need to create a gradually fading block of dots/"X"es covering a stimulus and as the dots slowly fade away, the stimulus become visible.

I tried created a transparent animated GIF as attached. But when i input them into Excel, it can't appear simultaneously with the stimulus. So the final effect i had was this gif appearing first afterwhich the stimulus appears. this is my code for the mask:
[code]_mask.gif (400x75)0,.4,03000*~Abused stray cats on street.0,.4,0rt:57*[/code]Any idea how i can get this to work?
Thanks a lot in advance for help! =)

[URL]https://email.smu.edu.sg/exchange/xilv.2007/Inbox/RATA%26Thesis/RE%20mask251414.eml/Xmask3.gif/C58EA28C-18C0-4a97-9AF2-036E93DDAFB3/Xmask3.gif?attach=1[/URL]

Since DirectRT treats animated .GIF files like video files, you may need to include the stimuli that is underneath with the .GIF.

If willing, it may be worth looking into [URL="http://www.empirisoft.com/support/showthread.php?t=4013"]DirectRT 2010[/URL], as the new version allows for transparent image overlays.

Also, the attachment you included can't seem to be accessed. Hm.