If you can't find the optimize for web/save for web option on whatever software you're using you can also look under resize or image size option. (Some photo software, except the newest stuff, doesn't have this option.) There you should see a section that will allow you to resize the image by inches, pixels, centimeters, or percent. Easiest thing to do is use pixels and bring the size down to no more the 250 pixels. If this isn't enough do a save as on the image, as a jpeg, and compress the image further. Between the two you should be able to get the image small enough.
Ceallach

Of course if your photo software has the option to save for web, definitely use that.