I'm presuming you don't want to physically reduce the dimensions of the image. If you do then click on the 'Image' menu and select 'Canvas Size'.
I'm afraid that's wrong! You need to select Image size to shrink the image, otherwise the image will stay the same size, but get cropped around the edges!
If you want to crop, then it is easier to use the crop tool than the Canvas Size.
For sambo: if you took the picture with a trillion mega pixel camera then you will need to reduce the dimensions aswell as the file size. The recommended dimensions are 640x480. This will more often or not reduce the filesize below the forum max of 80kB. If it doesn't you then need to go File Save as, and then you should be prompted with a sliding scale of quality. If you tick the image preview box you will be able to see the new size in kB.
rgds
Ol.