The quality of the image can be manipulated or "compressed".  If you want to reduce the "file size" but not the "image size", choose to store your image as a JPG file and choose the amount of compression you want before saving the image. JPG compression analyzes images in blocks of 8X8 pixels in size and selectively reduces the detail within each block.  At higher compression ratios, the block pattern becomes more visible and there may be noticeable loss of detail.

This image shows a bitmap (bmp) being saved as a jpg. As a bmp the file size was over 1 meg in size!
saved as a jpg with no compression at all, the file size is 275 kb.

Saved with a compression of 10 the file size is 107kb and no change in the image

Now we save with a compression of 20. File size gets much smaller with no real noticeable difference.

Finally a compression 60. There is no real reason to save with this high a compression, except to
show you what the image looks like pixilated. Your image may pixilated at a lower compression.
It just depends on color and other factors. A rate of 20 is good. If the image is to be printed I would
not compress over 5. This sample is for digital images and layouts only.

  To determine what resolution you will need for printed sizes,
see the chart below.  (from a printing company) 

So if you have a less than 1 mega-pixel
1 or 2 mega-pixel camera, Make Digital layouts!
They will be just as nice- just as sharp and just as appreciated-
as printed layouts would be. Cheaper too! No ink or printing needed.