To manage product images, you must be viewing a product. Once you are on a product page, make sure that you are on the General tab then click on Manage images on the right hand side of the page.
You should then see a page listing images for the product. There will be no listings if you have not yet uploaded images. Click on Add New Image to upload a new image.
On the next page, click on Choose File to select a file to upload, and then click Update. Once that is complete, you should be able to go back to the product page and see the image you just uploaded.