How can reduce a picture or image?

Photoshop

The question is the same with changing an image resolution or its format (reducing it to decrease the storage size of the image).

The quick answer is "use an image converter or editor to re-compress your photographs". You will loose a minimum in quality (not noticeable in all cases but very special ones) and get a 3-5 times smaller image. Common software are MS Paint, GIMP, Photoshop, Irfanview, ...

The long and more complete answer starts by stating that the size of the image is given by multiple factors:

- image format (jpeg, png, bmp, ...). For an optimized size of photographs jpeg is probably the best alternative. Changing a photograph to jpeg will make a big difference and will have minimal impacts on the image quality since the jpeg format was tailored to efficiently store color images.

- parameters of the image format (most important factor here is the compression ratio). Just by increasing the compression ratio, the size of the file decreases considerably. Using a computer to re-compress jpeg images is also a big leap in reducing the file size since the parameters of the jpeg library used in digital cameras trade size for memory usage and processing power. The computer has plenty of memory and processing power so that the jpeg library can compress better.

- image resolution. If none of the factors above work well enough for you, reducing the image size is the most effective way in achieving a smaller file. Beware that the loss in quality is one the downsides of this method. It is the most suited solution if you want to pack some photos in an email and send them. Keep the originals in the case you want to do some printouts later. Once the resolution is decreased there is no way to get the old photo back to the original resolution without noticeable quality losses.

For more information please have a look at the related questions.