WordPress.org

Scissors is available in the official plugin repository and can be installed directly from the WordPress administration interface. If you wish, however, to make a manual installation, follow these steps:

1. Extract the contents of the Scissors zip-archive you downloaded.
2. Upload the extracted scissors folder to your /wp-content/plugins/ directory.
3. Activate the plugin in the 'Plugins' menu in WordPress.
4. Configure the plugin in WordPress' media settings.

Automatic resizing of the full-size image at upload-time can be configured in WordPress' media settings. You can specify a maximum width and a maximum height in pixels for this to take effect (a width/height of 0 disables image resizing in that particular dimension).

Watermarking can also be configured and enabled in WordPress' media settings: Supply an image that you want to be embedded into newly uploaded images, specify its alignment, size, and margin, and you're all set!