The Media folder is where all media content such as photos, pdfs, docs, etc are stored.
To upload a new asset, click the upload button at the bottom of the page. Here you can upload one or multiple files. To do so, click 'choose files' or 'browse' depending on what browser you are using. You can also drag and drop files right from your desktop onto the upoader.
Once your files are uploaded, you can click on a file for more options.
On this screen you are presented with information about the file such as size and dimensions as well as the HTML for an imge or link so you can paste it right into a page.
To create a lightbox image gallery, just create a new folder and upload images into it. Then you can place it into a page or block using this syntax:
Replace 'galname' with the name of the folder you placed the images into.
When you click on a photo that is inside of a folder, you will see a textarea for inputting a caption. This caption textarea will not show up for all photos, only those inside a folder.
To resort the images in a gallery folder, just drag and drop the images into the order you prefer, top being the first. Like captions, this will only work on images inside a folder.
To set the size of the thumbnails, edit the gallery settings in config.php