It's not all about adding images - sometimes you'll need perform some other actions.
Here is how to perform some of these actions:
"Replace" lets you change your image, or in other words, swap out one image for another. When you press "Replace" it will then open a finder window prompting you to select another image from your computer.
To delete an image from a stream, you will need to confirm that that is what you really need to do.
First click on the image and select "remove" from the menu. This will remove the image from the image strip. However the image will still be part of the attachments area.
Don't worry if you accidentally press the trash icon on a colleague's image - the image will not be completely deleted. You'll be able to find it at the bottom in the attachment section. Then just drag the image into its proper spot.
If deleting is something that you would like to do, click the trash icon on the image in the attachment section. You will see a confirmation as well. This will completely remove it
If you have second thoughts, you can re-add it by using the Version History feature.