I have a photograph that must remain as a raster graphic. After I scan it into the computer and touch it up, what file format should I choose? There are so many.
Generally, you want to keep all your raster graphics in the highest-quality format possible before importing into Flash. One exception is when you use a tool like Fireworks to produce an optimally compressed image (say, with varying degrees of compression). If you do use an outside program to compress the image, just make sure you don't recompress in Flash (simply leave the default setting "Use Imported JPEG Quality"). Alternatively, if you import a high quality .pct, .bmp, or .png you can compress it in Flash until you're satisfied with the compression level. JPEGs are all right, but they always have some compression that may result in artifacts. GIF is not a good alternative because it can't have more than 256 explicit colors. Finally, simply changing the file format of an existing image will never make a graphic betterpotentially, it will only make it worse. Start with the best quality possible, and then bring it down as the very last step.
How do you determine how much one graphic is contributing to the final movie's file size?
If it's a raster graphic, you can explore the Bitmap Properties dialog box, which tells you exactly. Vector graphics are more difficult. Ultimately, you should copy the graphic into a new file and export an .swf of that file (through File, Export). Look at the file size. Sometimes it's not so important how much one graphic is contributing, especially if it's an important graphic. However, your concern should always be to not add to the file size unnecessarily.
I've imported a raster graphic and then used Trace Bitmap to turn it into a vector graphic. The result looks fine, but the file size has grown more than when the image was a regular bitmap. How can that bevectors should be smaller than bitmaps, right?
Not necessarily. This is a very common misunderstanding. It's possible to trace every pixel of a bitmap so that there is a tiny vector shape for each pixel. This will take more file space than the original bitmap. You can convert bitmaps to vector (with Trace Bitmap) anytime, but it only really makes sense when the nature of the image is appropriate or when you want a special effect. When Flash takes a very long time to execute a Trace Bitmap, it's a good indication that the file size might actually grow. (The delay is because the process is so complex.)
I have a fairly simple graphic (as an Illustrator file) that I would like to import into Flash. It's impossible to redraw in Flash, so I have to import it, right?
It sounds like a contradiction to me: It's simple but impossible to draw in Flash. Make sure you're fully exploiting the potential of Flash (read Hours 2, "Drawing and Painting Original Art in Flash," and 4, "Applied Advanced Drawing Techniques," again if necessary). If you have to import it, do so. Of course, export it from Illustrator as an .swf, or at least try to simplify the image as much as possible.