The Basics: To upload a file, visit the Uploads page. Two options are listed on the page: "Upload from your Computer" and "Upload from a URL".
Upload from your Computer: This option allows you to select a file that currently resides on your computer and upload it to GreyHen. Simply click "Browse", select the file you want, click "Open", and then click "Submit".
Upload via URL: This option allows you to take the URL of an image anywhere on the internet and upload it. To do so, find the image you want to upload, right-click it, choose "Copy Image Location" (or "Copy Link Location" if the image links to a higher-resolution version of itself), paste the result into the text field on GreyHen, and then click "Submit".
Tags: Whenever you upload files, you can choose tags which will automatically be suggested for those files. This is useful if, for example, you have a bunch of 3d renders to upload at once. Just type "3d" into the tag field, then upload your images - they'll all have that tag applied to them. Remember to clear the field before you upload something else, though!
Duplicate Images: Whenever you upload an image, GreyHen automatically checks to see if it already resides on the database. If it does, you will see one of two error codes: 1105 means that the image is waiting to be checked by a moderator, and 1106 means that the image is already on the database.
Clearing Your History: Clicking the "Clear" button above your results (or pressing the Escape key when a text field is selected) will wash away all of your results and tags, making it look like you just loaded the page. You can also reload the page to achieve the same effect - this method may prevent your browser hogging memory on slow machines.