If you check out this previous post, it has a plethora of links to awesome free product photography resources! Everything from how to build a light box to how to use a smart phone to capture beautiful images.
But sometimes (often), photos need a little something more than great composition and natural light. Sometimes they need a little tweak - brightened up, or cropped a bit. Photoshop is expensive, and you'll be glad to know, unnecessary for small edits like these!
PicMonkey is an easy to use free photo editing site. You can easily brighten, crop & resize your images, and apply other more advanced edits to the image. You can add text, to which you can adjust the opacity to create a watermark, or upload a second layer if you already have a watermark created you want to use. You don't have to create an account to edit photos.
Pixlr Editor is a little more advanced & similar to Photoshop. You can make more advanced edits to your photos, as well as the more basic edits like using opaque text to watermark your photos, brightening the image and cropping/resizing.
Gimp is a free program you can download and use from your desktop and offers the same features as both programs above as well as blemish removal, animation capabilities & many more options.
Whatever your computer capabilities are, these tools combined with the photos tips here can help you create stellar product photos and place your amazing products right in the spotlight they belong under!