I get excellent results with the XAT Image Optimizer. It's much better than Photoshop or any other tool I've ever tried for compression of jpg images with control over image quality. It's especially bothersome for Opencart to recompress my images when I have such an excellent tool for this purpose.
1. Change the English PHP file in catalog/language/english
2. Change the English PHP file in admin/language/english
3. Upload each to the equivalent place on your OpenCart server.
4. Delete your browser cache. In Firefox Ctrl+Shift+Del to get the menu for this.
5. Reload the pages.