Ideally, you should take your product images at the highest resolution you can. For those times when you just can't find the original image, or you need to increase the size of your images without losing quality, there is help. Lifehacker has posted a great tool for doing this called SmillaEnlarger:
Windows/Mac OS X: Enlarging images, especially from lower resolution source images, can be dicey business. Want to enlarge an image and you don't want it to look like an 8-bit video game sprite? SmillaEnlarger can keep things smooth and artifact free.
Photo by 512 Photography.
SmillaEnlarger is an open-source and portable application designed to help you intensively massage an image enlargement to keep it from looking jagged and filled with artifacts. You can select the level of zoom using the zoom slider and the location of the zoom via the selection box.
Once you have the selection you're after, you can begin tinkering with the sharpness, flatness, dithering, noise levels, and more. The preview function is quite speedy so don't hesitate to preview often to check how the various settings effect the outcome of your image.
When your enlargement is satisfactory, hit the Calculate button to render it—a process only slightly longer than the preview. From our testing, the results achieved with SmillaEnlarger are on par with other—usually pricier—methods.
If you know of any other tools—freeware, web-based, or otherwise—for easy photo enlargements, let's hear about it in the comments. SmillaEnlarger is portable freeware, Windows only.