If you have the slides saved as .jpg files, you can resize them with Picture Resizer.
But you first need to download the tool - when you click on the download link, make sure to save the file to your Desktop or to another suitable location. Do not run the tool directly.
Then drag and drop the images on the icon of Picture Resizer with your mouse and they will be resized. More info on the tool's home page.
(Thanks for donating, though it does not influence the tool in any way.)