It's probably an effect of how you've sliced the image.
Can you insert your Fireworks document into a post (click the camera icon) or upload it to a share? We can help you best when we can see what it is you're doing.
Personally I dont like using all these automated slice, dice and upload functions. Doesnt really create a compliant and nicely coded site.
What I do is slice up my Fireworks mock layout. Then I optimise the images. Then I right click on only the slices I want, and export to a folder.
I then use these sliced images to manually create my web site using Dreamweaver. I am in the process of writing an article on this method. See my site link :-
That's a nice start, Carlo, You'll need to fill in the details quite a bit. Especially in this region:
...Attention to slicing, positioning, colours and text must be given such that its components (slices) can be utilised in the final site. Slicing must be appropite for final site assets (make sure images are named and optimised. (see Photo)
Each of those sentences could be a longish tutorial in its own right.