A simple way is to download a free graphic editor from IrfanView.com. Then open your images with IrfanView, and under Images --> Resize/Resample, adjust the image sizes, and save them off with a different name. Resampling, as opposed to resizing, creates a higher quality image.
I usually embed the pixel size in the name, so I can tell which one is which. For example, if the original image was "my_image.jpg", and I resized it to 100 pixels wide by 60 pixels high, I would save it as "my_image_100x60.jpg". Then upload your resized images, to your "myPictures" folder on your web site. The Wizard's Image Picker will then be able to see them.
Just be careful to save it in the same format as the original. With IrfanView you can change the type (i.e. JPG to GIF), without realizing it. Irfanview is also useful for cropping, annotating, creating panoramas, slide shows, thumbnails, etc.. A very nice graphic editor to have on hand.