Well, in iPhoto, crop your photo to the most appropriate aspect ratio using the options in "Custom" field of the "Constrain" drop down menu. A box will appear and you can type in the eventual resolution you would like to achieve, such as 75 x 75 (MacRumors avatars). You will get a clear box over a faded version of your photo that you can stretch and move around anywhere you'd like, but it will always retain the correct aspect ratio. Then click "Crop" and then "Done." If your image is already the correct aspect ratio, then you can just skip all of that and move on to the next step.
Making sure the photo is selected, go to File -> Export, change format to JPEG, select "Scale images no larger than:" and type the final resolution you'd like once again (the same res you typed in before). Click export and tadaa.
If the exported image is still too big file size wise, you can open it in Preview and save it over itself with a lower quality JPEG setting.