Here are a couple of options for you:

1. You can use an image resizer to see if that works for what you want to do. Here is a free one - http://www.bricelam.net/ImageResizer/.
2. If you have Photoshop or PaintShopPro, you can resize them in these apps.
3. You can download Gimp.org (free) - https://www.gimp.org/. 
4. You can send the images to us at support@teamsideline.com and we can do this for you.