Lines in .gif images disappear in Flash player

I have a Flash application (CS3, Actionscript 3) that displays .gif images made in Fireworks. The images are simple line drawings. Everything works great in the development environment when I test the movie. When I deploy to the web and view in a browser the .gif images fail to display properly. Some horizontal and vertical lines become invisible. The lines that disappear are thin lines, and I’m guessing that the player reduces the size of the .gif and in the process has to throw away pixels.

Is there any way to tell the Flash player to display the .gif file without resizing? Or is there some other solution to my problem?