Here's one way to make them smaller under my old OS. I hope it's similar for your newer version:
Open the picture in Preview (might be your default image viewer).
Under the "Tools" menu, select "Adjust Size"
I usually change the size as a percent of the original, but you have other options. Also, lock the width and length together.
There is an informational area that shows the resulting size, so you can be sure you are meeting WtW's size limit.
Then, you will have to go to the File menu and save the new image. I usually save it under a different name so I don't overwrite the original.
I hope this helps. If you have more questions, please ask.