the picture size depends on the picture.
If they were saved as uncompressed bitmaps, they would all be the same size of around 10MB (if 16-bit colour), but saving them as jpeg's compresses them, and depending on how complex the pictures are, they will be different sizes.
Adding to what has been said already, which is quite correct: a jpeg at 3MP is approximately 1.2Mb. It varies with each jpeg, but that is a reasonable average. On a 650 Mb CD you are going to get around 540 of these jpegs if you burn them all in one session, or about 580 if you use a 700 Mb CD. 5 MP images vary even more but tend to stay at less then 2 Mb from what I've seen. This means about 320 to 350 images per CD.