"The file could not be uploaded. Please make sure that the file is an image and that it is under 100 KB in size. "
The system will only allow you to upload an image less than 100k in size. This is not to annoy you, but is for your own good. Large images cause the web page to load very slowly and this can be very annoying to your visitors. We recommend the following:
Thumbnail Images: Less than 200 x 200 pixels
Large images: Less than 600 x 600 pixels.
IF YOU NEED A WAY TO SHRINK YOUR IMAGES, DOWNLOAD "PIXresizer". (Only for Windows, won't work on Macs)
Instructions for resizing your images can be found here.