[robski]

To put a different slant on what Chris has said.

Factors that affect the kilobyte file size.

1) The number of pixels in the image.
2) The amount of compression. Two factors control how much an image will compress by. The first is the amount of fine detailed subject matter. A shot with a blurred background will compress better than a shot with loads of blades of grass for the background. Secondly is the amount of digital noise. Think of it as unwanted fine detail.
3) Use Noise reduction software or shoot at a lower ISO setting.

Using the above information it should be possible to get a very high percentage of your images under the 300Kb barrier with good image quaility and of viewable size.