by importing images into flash you will increase size of your final flash file. use of gif/jpg/png depend what kind of image you try to import, if image is like a photo it is better to keep it in jpg or png if you prefer. On other hand if the picture is kind of cartoon/drawing import it as gif.
There is also posibility to transfer bitmap graphic into vector graphic so it can free strech without pixaleting, but I can't remember the proces of transfer. this is only recomended for cartoons/drawing etc as they are made of areas of same or similar colour.

Hi

You can open any image file jpg/gif/png/ in flash. However, if you want it in transparent backgorund, i prefer to save the photoshop file as gif. Then you can open it directly in flash or use import. But be sure that you choose a transparent background (again, this time in flash) for this file.

I agree, I've never had problems with transparency when using .gif format and you can still get a pretty sharp image.

Use png format. Much better and smaller for photographic images with transparent background.

u can import images from flash mx, but if you want to export image from photoshop publish it in swf.

png is advised to use for transparent images.