BIMP is a simple and manageable batch image processor, that lets you do various things on multiple files.
You can use BIMP to create thumbnails, add prefixes and suffixes, replace spaces in names, rotate them, invert them, mark them with a text, apply diverse filters and graphic effects, text layers, and the exit can be redirected to an FTP server.
With BIMP you can also modify the size of images as you like, respecting or not the original proportion as you like. And as if all that wasn't enough, the program also incorporates options to convert files in GIF, JPG, TIFF, PNG, BMP, MIFF or TGA formats. And all these actions can be done on one or various images at once, and even in a combined way. The program is translated into various languages.