rounded rectangle problem

[Wicky]

hello,
I'm creating some h3 backgrounds for my website and for that I'm creating some half rounded rectangles. (where the h3 text comes in). First i make a background with no line and then a half rounded rectangle (via slice). But when I'm exporting the whole thing (gif) and test it by putting it on a another background there are some line-pieces visible. This sounds very confusing but when you see the file everythings clear (I hope)

Sander