Image Size for Gallery
We recommend that your images be no larger than 800 pixels wide and a resolution of 72 dpi. Images cannot be larger than 1MB each.
You can easily run a script in Adobe Photoshop to batch resize a large group of photos. Watch the tutorial below:
If you do not have Photoshop, you can use a free service to resize your photos like: http://www.picresize.com/