You can resize them from a photo editing program or here's an online free resizer.
Free Online Picture Resizer - Crop and Resize photos, images, or pictures online for FREE! Make your pictures about 600x800 pixels and save the new picture on your computer. Another mistake people tend to make is they select a picture, but forget to select "upload". After you have selected upload it takes a few seconds for your picture to be uploaded. Select "preview post" to see if your pictures are attached.