There are three steps to adding an image to an Opportunity profile.
From the Attachments Tab,
1. Select "Profile Image" (you will know it is selected because it will appear darker)
2. Then, to your right, click on "Choose File" to select the image from your computer (you will know it is selected when you see the title of the image.
3. Back to the left, select "Upload" to upload your image.
You will know you are successful when you see the green check box in the attachments and you see it in the Image Preview: