1. In Painter, open your file. You will want to create a mask for the portion of your image that you wish to make transparent. You can do this using Painter's masking tools, or you can select Effects/Magic Wand to select the area. You can add to the selected area by using the shift key with the Magic Wand tool. Once you've selected the background area, select Effects/Mask/Invert Mask.
2. Next, select File/Save As/Gif. Select the number of colors you wish your gif to have, then select the Transparency Output Checkbox. Click on the Preview Data icon to preview what your transparent gif will look like.