Hi Jane - just a quick note on colour depths & compression - if you reduce the colour depth of an image and then JPEG it, you actually make it bigger. JPEGs handle smooth, graduated areas of colour best, and trying to JPEG a dithered image with reduced colour-depths will make the file size much bigger (and reduce the quality of the image a great deal more than JPEG-ing alone).

Try saving your images out as JPEGs *before* reducing the colour-depth and you'll see a significant reduction in filesize *and* an improvement in quality. You should never really reduce the colour-depth of an image unless you're saving it as a GIF - and GIFs are best suited for line art/limited colour illustrations, not photos.

Cheers,
H.