|Free jpeg compression software is available here.
Free jpeg Optimiser for Windows
jpeg Optimiser can reduce the size on disc of image files. The files are saved as jpegs. JPEG compression, unlike, say, zipping, is not a loss-less compression. When you zip a file with zipper, you do not lose any information - it is loss-less. But jpegs are already compressed, and zipping them does not reduce their size on disc very much (Perhaps 10%).
JPEG Optimiser reduces the size on disc of jpegs, but it does so by reducing their quality. It is lossy compression. Wikipedia says:
"In computing, JPEG is a commonly used method of lossy compression for digital photography (image). The degree of compression can be adjusted, allowing a selectable trade-off between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality."
In other words, you can reduce the size on disc of image files quite a lot by reducing their quality, without making the images look noticeably different. (They quality is reduced, but up to a point it isn't noticeable). The original images that are compressed 10:1 are probably bitmaps or tif files.
In the example below, the original image is drastically reduced in size from a bmp image. The original image is 3.2 Mb on disc. Changed to a jpeg with 100% quality it is 383K on disc (12% of original size). Compressed in jpeg optimiser, it is 26K. Even when compressed to a fraction of its original size, it is still acceptable for some purposes. But, and this is a small but, the image shown is 25% of the original size - not resized, just shown here on the page - so it isn't easy to notice any defects in quality. At a zoom of 150% I noticed some blemishing.
So you can reduce the space needed to save your image on disc, but there may be a cost.
Images compressed by jpeg Optimiser may be indistinguishable from the original except they use less disc space (not smaller in width and height!). JPEG Optimiser is used to reduce the size on disc of image files so they just fulfil their purpose (and no more). For instance, the image might be fine on a webpage and small enough to load quickly. It might not be good enough for, say, printing. This is more likely when an original image is a a jpeg. A bitmap or a tif compressed (saved as a jpeg) may be generally acceptable. A jpeg compressed to the extremes so it looks good-enough on a web page might not be good enough for other purposes.