I hope this doesn't sound stupid but it might be very simple. I noticed on clement's game page for Luor he used pics for the title and other stuff, that include the background (the blue rings). Though when you right click the image and open it in a new tab the background is simply white. That means somehow he made it so it takes the background of the page. My question is how do you do this?
Transparency settings. I think it can only be done with .gifs and .pngs. If you have a more complex program than MS paint for creating images you can usually edit the settings upon saving to select a colour to be transparent, and some editors allow transparency to be set whilst actually creating the image. Depeneds on the program I believe.
It is possible with MMF2's editor. Any frame in an animation or any image made in the image editor can be exported from MMF2 as a PNG with transparency (including alpha channels) completely intact. In the top right corner of the window should be a save icon. That will let you export the image as a file.
Use the magic wand/select tool to select the area you wish to be transparent and the press delete. The Grey and white chequerboard effect tells you what is transparent. There's no doubt other ways, but i haven't got time to find them all out at the moment Hope that helps!
The MMF way Jon described is probably infintly easier for basic transparency effects i reckon.