A file can be saved as a GIF simply by choosing File > Save as > Save as type > CompuServe Graphics Interchange (*.gif). However, I find my results are much better when I use the optimize feature when changing the file type to a GIF. The main reason the results are improved is that when the type is changed in the "Save as" dialog box the default settings for the file type selected will be applied. This can result in various defects including color changes, posterization, loss of transparency and larger file sizes than necessary.
When a file is optimized the settings can be customized to fit a specific image. Some of the options available include transparency settings, the number of colors used in the final image, the palette used as well as other details. Once your file is ready to optimize be sure to save it as a PSP file first just in case you need to edit vector items or individual layers later. Then select File > Export > GIF Optimizer.
In the GIF Optimizer dialog box on the Transparency tab there are several options for making the image transparent. I think the most simple method is to use the Existing image or layer transparency. This requires that your image is already set up to look as you'd like it to appear when it is saved (ie: there must already be transparent parts in your image).
On the Partial Transparency tab I typically use a very low percentage opacity like 1 or 0. The dithering option adds noise to the image because it blends the available colors to approximate the original image colors. I prefer to use my original colors as much as possible so that is why I use the nearly full transparency option. On the blending partially transparent pixels option I usually choose Yes, blend with the background color. This helps prevent jagged edges. Just be sure to choose a blend color similar to the background color the image will be displayed upon or a halo may be visible.
A GIF can use up to 256 colors. The size of the file is directly impacted by the number of colors that are used. When a 24 bit image is created in PSP there are 16 million colors available. When those 16 million colors have to be condensed down to 256 color or less there can be some quality loss. This is most aparent in gradients and photos. In these instances I find it best to use a JPEG. However, if you are saving an image for use on the internet then sometimes a smaller file type is worth a little color loss and a Gif is a good way to go.
On the Colors tab there are four palette choices: Existing Palette, Standard/Web-safe, Optimized Median Cut, and Optimized Octree. I've found for photos and images with lots of colors that most of the time the Optimized Octree option works best. When I'm really worried about color loss I use all 256 colors and 0 on dithering. When I'm most concerned about file size I try to limit my colors while watching the preview on the right. The Compressed file size is listed above the Colors tab on the right and as you adjust the colors the file size will increase or decrease.
On the Format tab I prefer to choose Non-interlaced. I don't like watching blurry grahics come into focus so it's really just a personal preference on that setting. Although you need to be sure to choose a version for your GIF that supports transparency.
I always take a look at the Download Times and that helps me determine if I've done enough to reduce my file size. If my file is still quite large then I go back and adjust my colors until I can get it a bit smaller if necessary.
I always recommend reviewing all five tabs prior to selecting "OK". Be sure compare the views of the original image in the window on the left with the preview window on the right, and zoom in and out on the preview to make sure your finished image will look as you'd like. The most important thing is to make a back-up prior to exporting to GIF and then experiment until you get it just right.