Help! How do I remove a WP dissolve pop up window?

I had my site done for me, and am not pleased with some follow up work I’ve had done. I would rather correct it myself if I can so that no further issues are created:
They have put a ‘register’ window that pops up on the first visit to the site. I actually don’t want to remove it, but want to replace the ‘register’ stuff with an image instead. If I can’t do this, then I would like to remove it all together.
Can anyone tell me where I would find the file that governs this?
So you have a better idea of what I mean, the site is http://www.mobiusmanitou.com
I’d love-love-love to get this fixed asap, as I think it is a bit arrogant looking to have that there.
Thanks in advance!
Mobius.