To upload media on the site you can use the activity feed on the home page, or from within your profile, or any group you are a member of. The site supports the upload of images and video and documents. The various file types supported are indicated on the screen when you click the image landscape icon which opens the uploader interface from where you can select your file.
A single click anywhere in the activity field will display the media upload button (image landscape icon), and a popup selector of groups into which you can choose from. The default choice is for the upload to go into your default profile gallery. Follow the usual "select file" uploading instructions and you should be able to figure out the rest.