There are at least two ways to reduce the file size required to store an image, and thus reduce the time required to download it. The physical dimensions (pixels horizontally and vertically) can be reduced, or the degree of compression can be increased. Unfortunately, JPEG compression is "lossy" in the context of image quality.
Below are some examples of the trade-offs between image quality and file size. In each case the resolution has been kept at 600 by 400 pixels, so the only two variables left are compression and quality.
This particular photograph is very compressible because of the large, smooth gray background and soft clothing. Most photos, especially those with sharp edges between contrasting colors, will not fare nearly as well
This is a cropped section of a 205,472 byte, 642 by 864 pixel image file of model Paulina Porizkova, who escaped Czechoslovakia when political tyrants were in charge.
The first image was saved at maximum quality and the resulting file uses 77,455 bytes. Note the extremely smooth skin tones and very clear edge between the arms and the gray background:
The second image was saved at a quality marked "80%" in ThumbsPlus, and the resulting file uses 33,016 bytes. Minor compression artifacts can be seen along the outside edge of her right arm, and some discoloration on the fingers of her left hand. Still quite an acceptable image:
The third image was saved at a quality marked "60%" in ThumbsPlus, and the resulting file uses only 20,141 bytes. The artifacts along her right arm are more pronounced, her left fingers are quite blotchy, and the skin tones (look between her eyebrows and around the nostrils) are not so smooth:
The fourth image was saved at a quality marked "40%" in ThumbsPlus, and the resulting file uses 14,277 bytes. The edges of both arms and both hands suffer from compression artifacts, as do the background and the white sweater, and there is annoying discoloration around her nose and eye. Her eyes are not as compelling as in the first image. This image is barely usable on-screen, but you wouldn't want to print a hard copy:
The fifth image was saved at a quality marked "20%" in ThumbsPlus, and the resulting file uses only 8745 bytes. Distinct "posterization" occurs in both the background and the skin tones, and the resolution of her hair is poor. This is clearly not an acceptable compromise!