Jorge,
I had the same problem as you describe and my customers hated the way my disabled images looked in my command buttons. My solution (warning: non-standard) was to use the same image I used for my Picture property in my DisabledPicture property. IMO (and that of my users), VFP does a good enough job of making a disabled button look disabled without having to change the image. IMO, the result is a much cleaner, professional GUI.
Tip: One would think :) that you could use the same image file for both the Picture and DisabledPicture properties - but, for some reason(?), you can't. If the file names are the same, VFP will try to create a disabled version of your DisabledPicture image anyway.
My solution was to make a copy of all my standard command button images with a "D" suffix so that for every image I had 2 identical image files. For example: If I had an image file called FileOpen.bmp, I created a copy of this image file called FileOpenD.bmp.
I used the *D duplicate image filenames in all my command buttons with DisabledPicture images.
Hope this helps,
Malcolm