You can edit product images in your magento admin panel
CATALOG > MANAGE PRODUCTS
Here you can search to locate the product you want to add images too
When you have located and selected the product go to the "Images" tab down the left side
To add new images first select the "Browse Files.." Button
Select the files you want from the popup and click okay
Then to add them to the site click "Upload Files"
These will then appear in the table with a preview
Make sure you main image has the radio buttons ticked for "Base Image", "Small Image" and "Thumbnail"