A portfolio is a group of works created by you, the artist, which feature at the top of your profile.
Collections are groups of images curated by you and your fellow artists and art lovers. A collection can include work from a single artist or from several different artists.
Click on "Add a Collection" link in the Collections section of your Profile page or click on "Collections" from the top menu and then click "Create a New Collection."
Click on any image to get to the Image Detail page. Below the image, click on the "+" sign to add the image to an existing collection, or create a new collection for it.