I assume you mean on your hubs? If so it's actually pretty easy.
1) In the edit mode on the hub you want to add photos to look in the upper left hand corner. You should see a section title "Add More Capsules" find the one that says "Photo" and click it.
2) Scroll to the bottom of your hub (still in edit mode) until you see the empty capsule called "photo" and click "edit"
3) In the new window that opens look for the line that says "Add photos from:" and then either select "My Computer", or "The Web".
If you chose to get the photos from "My Computer" you'll see button appear that says "Browse" click on it and then navigate to the picture on your computer. Once you find the file double click on it, or click the button in the lower right that says "Open" and then click "Load Images"
If instead you find a picture you like online you'll need to put in the exact file path to the photo. It will look something like this http://mywebsite.com/mypicture.jpg
The easiest way to get that information is on the picture you want to use right click it and then in the sub menu that opens click properties and look for the line that say gives a file name that ends with an image extension (.gif, .jpg, jpeg, png are the most common ones you'll come across). After you've put the file path to the location of the image in click "Load Images"