There are two icons associated with image upload:
The first lets you pick an image from your computer or phone. The second lets you use a description and file path. For most people, the first icon will be the easiest to use, assuming your computer or phone implements a good file/image picker.
Two other ways to insert an image are:
- copy the image to the clipboard and paste it into the message
- drag and drop
All of these depend on your browser working correctly. So try a different browser, especially if you're using a substandard browser -- and doubly especially if it comes from microsoft.