I use Adobe Fireworks. I have a file called thumbnail.fw.png which has multiple layers of images. I replace certain layers with other images. Like the very center image is usually the product I am going to review.
I either take a photo of the product, or find a nice high res transparent png image of the product. Then I import the image for the center, add a glow or drop shadow effect to it, change the descriptive thumbnail text and export as a lower quality .jpg so YouTube doesn't complain about the file size being too big.
My thumbnail template is 1920x1080 to get the perfect resolution size.