To add images to your products please follow these steps
1) Log into your magento admin panel
2) Navigate to CATALOG > MANAGE PRODUCTS
3) Search from the product in the table that you need to change
4) In the side panel select "Images"
5) Click "Browse Files" to locate new images to add
6) When located click "Upload Files" to push the files to the server
7) When uploaded select the radio button for Base Image, Small Image and Thumbnail for the main image you want to use
8) To remove any old unwanted images tick the "remove" tickbox and then "Save"
Always ensure you save when complete to avoid losing any work