If you want to update an existing image with a new version of the file you can replace the image via the Shared Content screen. Here's how;

1. Locate the existing image in the Shared Content screen

2. Right click on the image

3. Select 'Replace file' 

4. Drag and Drop or upload the updated image via the 'File replace' tab

5. The updated image will then replace the original image in the Shared Content folder

6. Any course that used the original image will then be updated to show the new version when the course is next Previewed/Published