How do I reduce a picture to less than 50kb and less than 500 by 500 ratio?

Hmmm, smaller image should be smaller size.

(Unless the pixel dpi is increasing to compensate...
for a computer screenm 72 dpi is a good bet. but i can't see where iphoto does that)

Have you tried share > email > small?

Paul,

iPhoto won't downsize your photos, since your original would lose much quality if you could do that. Just crop to best suit the aesthetics of the photo. Then to resize for the website, do File > Export > File Export to a Desktop folder. Instead of "full size" select "scale photos no larger than" and type 500 pixels in one of the dimension boxes. (Choose the larger dimension - width for landscape photos, or height for portrait.) iPhoto will automatically constrain the proportions and enter the second amount. Only the exported copies will be resized. Upload those from the Desktop to the website, then trash when finished.

Regards.