I do not know about a Canon S20, but I do use a Canon Powershot Pro 90IS. With approx. three months shooting experience with this camera, I ALWAYS go for the higheat quality; highest no. of pixels and RAW mode (saves the file uncompressed). I add a little saturation as the Pro 90 seems to lack colour depth sometimes.
Using my current available memory, I can get 60 shots uncompressed or 90 shots in 'best' jpeg mode, I prefer the 60 shots in 'best' mode every time. With a digital, you can always delete the 'wasted' shots on location to give you extra shots at the end of the session.
I looked at your camera on the Canon web site and I see that the higheast resolution is 2048x1536 with super fine jpeg. Do not shoot below this and NEVER use the digital zoom. Use the optical zoom only, as digital zoom just invents pixels to put in between the real ones and tends to make the images fuzzy. The supplied memory card (8MB) will hold just 6 images at max. resolution. Better to come home with 6 images, of which 2 may be suitable for a.net, than 20 really low quality ones. The higher the pixel count on the original, the better the finished photo will be (assuming that you resample to 1024 pixels wide for A.net).
I never use a monopod or tripod to shoot aircraft, as the ones I shoot are usually zipping past at 170mph while rotating.
Learn to pan and resist the temptation to stop panning as you squeeze the trigger.
I look forward to seeing you pictures on A.net.