This is how I do it in Photoshop Elements:

Open the shot you want to edit and click on 'image'.  From the drop down menu select 'resize' and then 'image size'.   You'll find a box that says 'resolution' which will default to 300.  Change that to 72 and then change the length to 640 if the shot is in landscape format or height if it's in portrait.

You then need to save this re-sized image to a different folder to the original.  Click 'save as' and then save it to the new folder.  When you do this you can select what level of quality this is.  You'll need to select about 5 for a sub-100k file size.  Job done.