GIMP is great and I use it all the time, but when it comes to batch image processing on Linux, nothing is more handy and simple to use than XnConvert. Although not an open source software, this batch raster graphics editor comes for free without any limitations for private use, and works in all platforms and architectures. You can get it from the official website as a complementary part of XnView (it’s standalone). Here comes a tutorial on how to use this simple yet powerful tool on Linux.
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