Access Your Virtuemart Configuration
Go to Administrator >> Components >> Virtuemart >> Admin >> Configuration
Access the Downloads Tab
Check the box next to "Enable Downloads" and click "Save.
Open an Existing Product
Go to Products >> List Products and click on an existing product.
Have a Virtuemart Product Open
When the product is open, you'll see a new link on the left under Products. This new link will be called "Add Image/File". Click on it.
Upload Your File
After clicking on "Add Image/File" you'll be taken to a screen like this where you can attach images to your product.
Seeing Existing Files
You can always check back and see which files are assigned to which products by going to Products >> List Products and clicking on the "Media" column.