Let me back up and explain a little better....
I've been working on sleeve for a CD in Inkscape, which is a freeware vector-based drawing tool. I am placing text on top of a graphic, so I have to make the graphic about 50% opaque so the text can be seen.
With inkscape, I can only save as a native file, or import to a bitmap. When I import to a bitmap and open the file, the graphic remains light, and the text is still readable.
Now, when I open the bitmap in The Gimp and save as a jpeg, I get the same result as above. When I open the bitmap in Paint.net and save as a jpeg, the graphic returns to it's original state (i.e. darker), and the text cannot be read.