.welcome is a file that is automatically read and displayed on the first page of each directory when in browsing mode. You can edit its content either via the interface using the "Edit .welcome" button when authenticated as admin, or via your favorite text editor and then uploading it to the directory where you want the text to be displayed (this directory must be within pictures_dir).
The beauty of the .welcome is that you can also include HTML code directly inside, thus making the possibilities endless. For example, you can add links (like shortcuts to directories, you can include images (ie: picture of the month),etc but as an example may be better than thousands of words, copy/paste the following code to your .welcome (don't forget to adapt the link to your site)
Below is a link to the last commented pictures: <a href="index.php?lastcommented=">Last commented pictures</a> Below is a link to a specific directory: <a href="index.php?dir=path_of_a_directory">Direct access to a directory</a> Below is an image inclusion: <img src="index.php?display=path_of_your_picture" />