This changed in Elements 3.0 you now set the alt tag in the Image Resource, this means every time you use the image in the project it’ll always have the same alt tag. We think this is a much better way to do it
you can switch to the “Custom” option for images and this will include an Alt tag option below the source URL )this is probably more like you’re used to working in Classic):
You can add remote images to the Resources area, right click the heading and choose “Add Remote Recourses”.
This allows you to paste in one or more external URL’s, then you can add alt tags in the Media Inspector like above. And yes, the resources are not added to your project, they are still remote. See Docs for more information.