The industry standard when it comes to graphic design and image manipulation is Adobe Photoshop, however the full featured version is quite expensive.
Fortunately you don’t need to go searching for illegal copies of the program, as there are some great free alternatives available.
Artweaver is program that has many of the same features as Photoshop.
You can work with multiple image layers, adding text and filters with the ability to export to most of the widely used image formats.
There’s also a paid version of Artweaver, however the free version will be more than enough for most. Follow the download link at http://artweaver.de/products-en/artweaver-free/