Change the Size of a Picture

I use "Microsoft Office Picture Manager" File/Export and choose new settings (e.g. make picture 50% of original size).

"Many common picture editing programs allow you to change the quality level of a JPEG picture when it is saved. When using the program, look in the program's Save As dialog box for an option to change the compression or quality level. The lower the quality (or higher the compression), the smaller the file will be when it is saved. "

Source: http://windows.microsoft.com/en-US/windows-vista/Change-the-size-or-resolution-of-a-picture