Download Shrinkpic; it automatically reduces the photo file size sufficiently for e-mails and forum postings. If necessary, it can easily be switched off when you need to send full sized photos. I’m sure there are probably many other photo reducers available, but this has always worked well for me.
I’m by no stretch of the imagination a computer wiz, but I did find this on our computer. If your pictures are stored in windows photo gallery. When you open the photo there is an “Open” drop down menu. Click that and and select “Microsoft office picture manager” then click “edit picture” then “re-size” then “Pre-defined width X height” and there will be a drop down menu with several sizes, large web, small web etc.which will change the size to whatever you want.