The best thing you can do is lighten the picture part of the image in some photo program and leave the text as dark as you want. If you are using a .png image, be aware that IE6 makes transparent areas grey. So if you can, have a background color in the image even if it is white.
As Marshall indicated, using an image editing program and reducing the opacity should allow you to 'lighten' the image to any degree that you like, and that darker text should lay right over top very nicely.
I usually use (or convert) .jpgs (.png are not ready for 'prime time' yet). Don't use a lot of .gifs when working with my own images. What is the background color of the .gif - does it have one or is it fully transparent already? I would suggest continuing to practice and work with various options.