First, I would recommend using Special:ThemeDesigner, which has a transparency slider. I think the lowest it goes is 80% opacity, so if you need the page to be less opaque (more transparent), I have several transparent images on this page at my sandbox wik. Just click on a link to get the URL needed for the CSS.
If you want to make a tranparency image yourself, I don't know if MS Paint can do it, but I'm sure GIMP can, which is free to use. The way to make the image is to create a small, blank image (10x10 pixels or so) and fill it with black (or whatever color you want). Then on the layers window, you'll see Opacity with a slider next to it. Just adjust the slider to whatever percentage you want, then save the image as a .png file.