There are many ways to store photographs so that you can link to them and use them in ArcGIS Online. The most popular method is probably using Flickr, Google Plus, and other photo sharing services, as I documented in this set of guidelines. However, either because their institution prohibits the use of photo sharing sites or for technical reasons, some educators prefer to store their photographs in ArcGIS Online.
Let's say you have uploaded some photographs to ArcGIS Online by navigating to "My Content" and then using the "Add Item" function. How, then, do you link to the photograph once it is there in ArcGIS Online? You cannot link to the URL that is at the top of the metadata page for that photograph. Rather, you need to navigate to the bottom of this page to get to the actual URL where the photograph is located, as shown for a photograph I uploaded from a field trip I was on with educators in New Zealand, highlighted in yellow, below:
Once you have the URL, you can create a Map Note that links to that photo (shown below and in the map linked here), you can use the photo and other similarly-linked photos in a story map or other web mapping applications, and you can use them in other ways.
Hmm.. working with these images and map makes me want to take another field trip back to this spectacular landscape! Give this photo technique a try!