Shouldn't happen with PNG, unless perhaps the .png image uses alpha channel opacity. But it's a common problem in IE when fading .jpg images that have any 100% black pixels in them.
You can edit the image in a good image editing program like PhotoShop, Paint Shop, The Gimp . . .
Colors > Adjust > Levels
Use the RGB channel and (using the slider or just typing in the number) adjust the output 0 level to like 3, or 7, no more than 9. Some trial and error may be involved.
If you want more help:
Please post a link to a page on your site that contains the problematic code so we can check it out.