this is not a programming issue - you need to learn how to manage your graphics yourself - there are a many great apps that will do this. Some are free, some are paid. They all help you resize - change the DPI, crop, etc. These are things you should be doing with your own pictures prior to sending them to your cart files.
You can easily do a search for 'watermark graphics' and you will find them.
Here's a short list:
Free image editors – that will add watermark
Google Picassa
https://support.google.com/picasa/answer/106193?hl=en Gimp (a free graphics program) adds either a graphic or text watermark. There are lots of instructions and videos on the internet that tell you how.
http://kgiron.blogspot.com/2009/07/adding-watermark-using-gimp.htmlA free online service that you can do one graphic at a time
http://www.umarkonline.com/If you have Microsoft Publisher -2010 or later it adds watermarks
Here's some great software you'll have to buy, usually has free trials
Snagit from Techsmith – you’ll have to buy, has a free trial – great program
http://www.techsmith.com/snagit.htmlAnd of course, Adobe Photoshop - they have software and a cloud service - both cost money.
Hope this helps get you on the path for good graphics handling, again something you need to be responsible for.
Lee