Glimmr Canvas-Based Drawing
version 2/101030 by Erik Temple
Chapter: Producing images for use with Glimmr
Full instructions for producing images are well beyond the scope of this documentation, but a few notes and links will be useful. For almost all purposes, you will want to produce your images in PNG (Portable Networks Graphics) format. PNG graphics (file extension .png) can contain an "alpha channel" that defines transparent areas of the image, allowing for shapes that are not rectangular. They can be produced with Photoshop, of course, but also with many other programs, including the free, cross-platform GIMP software (http://www.gimp.org). Here are some sites that might be of use to you if you are unfamiliar with PNG graphics (Inform tends to mangle URL spacing in documentation, so you may need to edit these by hand or open the extension itself and copy the links from there into your browser):
http://www.axialis.com/tutorials/tutorial-misc001.html (Photoshop tutorial)
http://www.libpng.org/pub/png/book/chapter04.html#png.ch04.div.5 (GIMP tutorial)
Avoid making your images too large. Images should be sized no larger than you want them to appear onscreen.