RIOT (Radical Image Optimization Tool) [free] might be a right tool for this, which reduces any image (JPG, PNG, and GIF) considerably, without any significant quality loss. The program is simple to use, just download (1.14Mb, v.0.21) and install it. Select any image by clicking ‘Open’ button. Input any size (in percentage) or drag the ‘Quality’ slider to an acceptable level (you can view it in preview pane) and save the image.
There are some other options are also available with this tool such as rotating, re-sampling image, removing metadata (EXIF), etc, this further enhances the performance of the tool. More details and download at http://luci.criosweb.ro/riot/download/