If you are using Windows, go into your pictures, then click on one, then go to the top of the screen and choose email. Click on email and a small box comes up and asks what size you want to email. Pick the size you need then email it to your self or your site. If you email it to yourself in the correct size you need, then you can just copy and paste it.
You can also upload the pics to Flickr. Then when you click on your pic in fickr you get a screen with the pic and "all sizes" at the top. Click on all sizes and pick the one you want there.
Flickr is free for a certain number of pics per month.