I haven't used the one Sharon uses ... I use this one. Used to be that Windows had an Image Resizer in their tools, but it didn't move over when they went to 7. I don't know what operating system you have, so am going with one that I think works on any. What I like about the one I have the link to - is after you have an image on your computer (or any device attached to your computer) you can simply right click on the image and select "resize pictures" ... see screen shots in the link. You can also select multiple pictures, then right click and resize all at one time. I have mine set for a custom size and it automatically does all pictures to that size unless I change it. (I have it set at 640 x 480).
http://imageresizer.codeplex.com/
If you install this, you won't see it on your computer except if you go to a picture and right click - and the option to resize the image will be added to what you can do. Nothing else was added to my computer when I downloaded this - didn't have to click to not install a tool bar, etc. BUT always watch for those things when you download, and I won't promise they won't have changed anything.
Sharon saves the new images in a different folder ... I keep them in the same, but they have added info after the name, so I can see immediately which they are.
Being resized already, they'll also upload much faster to PB. All you have to do to share is upload and get the image code.
Good luck on whichever one you use. If you use this one, I can help. And I'm sure if you get the other, Sharon can.