OK, here is what the free app IrfanView does on reducing size.
Here's the initial 8MP (2448 x 3264) file, from a simple P&S camera (PowerShot SD1100 IS, ca 2007):http://hwstock.org/qual/NGA_original.jpg
Now here are images resized to 900x1200, at various qualities:http://hwstock.org/qual/NGA_91.jpg
(91% jpeg quality, 395 kB)http://hwstock.org/qual/NGA_80.jpg
(80% jpeg quality, 256 kB)http://hwstock.org/qual/NGA_65.jpg
(65% jpeg quality, 184 kB)http://hwstock.org/qual/NGA_50.jpg
(50% jpeg quality, 148 kB)
You really have to zoom in to see the differences-- e.g., look at these cracks, at 91%
Save these two tiny fragments, and zoom in with an editor to see the differences.
EDIT: actually I just artificially increased the number of pixels in the zoomed images, so the differences will be more obvious, without requiring people to reload anything. This won't show until my and Sp's servers propagates the changes.