What size should I upload?

Larger than the final image size within your site. The image you upload will be copied and resized automatically as required by specific templates. 

How to secure files from public access

By checking the secure option on the file upload dialog window, the file you upload will be stored in a location that is not publically accessible.

This is useful for using the Library as a file respository in combination with the Library user role. This role allows a user to be created with access only to the Library.