Under unix, there are lots of tools available for this. ImageMagick is one very nice package for doing this. GIF Converter is a good one for the Macintosh. There are comparable packages for Windows.
In general, all your images will need to be the same size. If you need to resize images, it is best to make the images too large and then reduce the size (enlarging images can cause serious pixelation effects, while reducing can allow for anti-aliasing and color-map corrections).
If you generate PostScript files for your frames, these usually can be scaled without distorting the image prior to converting the image to a bitmapped .gif file.
Created: Jun 01 1996 --- Last modified: Jun 01 1996