All depends on what eventual file size you are looking for, win xp will compress photo's for you automatically
just right click on the photo in windows explorer and select "send to mail recipient" you will then be given the option to change the file size, select what meets your requirements and click ok,
Your email client will then open up automatically with the resized picture already added as an attachment,
just save the attachment to any location on your drive and then go open it up to see what it looks like and see what the file size is, I use this method all the time for email/uploading pictures and it works fine for me.
HTH :)