I've done this by adding a separate directory inside the normal images directory, as kobra suggests.
I have a lot of products that are better with images embedded, to aid the description whilst also making it more interesting for the site user. To save that extra images directory getting too full, I will categorise the extra images in their own directories by product types. This will also aid me finding/replacing them should the need ever arise to update them and also keep the site structure neat and tidy.
Just add the directory locally, then add the photos locally, then FTP the whole directory with which ever program you normally use, like FileZiller for example. The new directory will be created for you at your site by the FTP program and also be populated as you upload them.
I find this also helps as I am a member on some product specific forums, so I can link the photos/pics directly rather than particularly linking the product itself :happy: