You squeeze them REALLY hard
Seriously, if you are asking you need a program to do it. Here is one of the greatest freeware (no ads, no spy, no nags, no anything) progams of all times: click for IrfanView website
For this purpose, just open your picture with IrfanView and go to Image..resize/resample. enter the size you want in pixels, click resample and select Lanczoz. Then resave the picture as a JPG.
If you want to be kind to dial up readers, you can adjust the physical size of the file with the JPEG save options. Its actually called "quality" but basically, the lower the quality, the smaller the file size. For web pictures, you can often go as low as 60% with no noticable effexts. Just look at your saved pic to see if it looks good. If it does, try lower quality settings until you don't like it and go back up to the one before.
