Unfortunately, one of the things that Microsoft removed from Office 2013 was the Microsoft Picture Manager that allowed you to easily manipulate images.
You can still install it by following the information in this blog post:
However here is another way to solve this issue.
The first step is to create a “mapping” from the SharePoint Online Picture Library to your desktop.
Start by navigating to your Picture Library in SharePoint Online. Once there, select the Library tab in the top left of the page to reveal the Ribbon menu.
Towards the right hand side of the Ribbon Menu in the Connect & Export section you find a Open with Explorer button.
You’ll then see Windows Explorer open and in there you will see the files in the Picture Library as shown above. You have now effectively ‘mapped’ a drive on your local desktop directly to the SharePoint Online Picture Library.
You can now manipulate those files as though they were on your local desktop. Beware however, that are still working with the files directly from Office 365. This means they will typically have to transferred down locally, updated and then saved back to Office 365 all across your broadband connection. If you have very large images (many megabytes for example) this may mean things work slower than expected. Yet another case for better bandwidth.
If you select the Manage tab in Windows Explorer you will see that there is some basic manipulations that you can perform such as rotating the images.
If you need more functionality when working with images you can now use just about any program and simply point it to the newly ‘mapped’ SharePoint Online Picture Library on your desktop.
In the case above I’m using the free Windows Live Photo Gallery product from Microsoft.
Now I select the image that I want to rotate and then the Rotate Left button from the Ribbon Menu.
You will then see the program working on the file. In this case there is clock icon next to the image and the status bar at the bottom says it is updating the file.
Remember, this may take longer than you expect, depending on the size of the image, because it needs to be downloaded to the local desktop, manipulated and the saved back to Office 365. That’s why you should really ensure your images are only as large as they need to be.
The process will complete and the updated image will appear in your application.
If you now refresh the browser page that also displays the SharePoint Online Picture Library you also now see that it has updated with the changes made to the image as shown above.
“Mapping” a drive like this to SharePoint Online is not a prefect solution. It can have challenges at times, typically do to the desktop and broadband connection. A far better option would be if Microsoft incorporated even some basic image editing features (re-sizing, crop, rotation, etc) directly into Office 365 so you could do all this in a browser and without the need for the image to be brought down and saved back.
Hopefully, they have that on their ‘to-do’ this but hopefully the information provided here will let you get the job done.