Add mages to product
Images *To add additional images and/or media like Audio, Video, Pdf, Archive files go to the 'Media' tab and click on "Plus" button. The Resource library popup window will appear.
In order to set an main image for your product simply click on the "Plus image" and a popup will appear, click on first Tab in the popup, images mapped to current product will appear, change sort order of image and click on Save Sort Order button.
Maximum size of an uploaded media files depends on your server configuration and Resource Library Upload Max File Size setting in System > Settings > System.
See how to Add YouTube video as resource to product