Use photoshop. Go to the type of file and save a a JPG (JPEG). It saves it as a much smaller file and you can say what quality that you want. For example, i made this picture smaller by saving half of the maximum quality (there are 12 levels, I chose level 6)

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-Justin

91 560 SEC AMG - other dogs dd
01 Honda S2000 - dogs dd
07 MB ML320 CDI - dd
16 Lexus IS250 - wifes dd

it's automatic.