Content Block Edit Images
I am actually looking out to replace the image which has been added in the content block.
1. Click on the page-looking icon for the "link" option. This will open the file manager.
2. Concrete includes a built-in image editor, so select the image you want to add, and choose "edit".
3. Once done editing, you should be able to choose to add it.
However, it looks like this will only link to the image, not actually display it. If you're using the main content block, the Redactor interface should have an image icon in which case you can click on the search icon to open the file manager (and from there edit the image).