Is batch conversion from JPG to BMP possible in Gimp?
There is a plugin available for Gimp - Auto convert and resize images
It automatically converts and resizes entire collections of images. The complete path of the images is read by the script of the text file to convert one complete path for every line of the text.
Try Easy Graphic Converter is an efficient tool for mass image resizing, renaming and conversion, It's amazingly easy to use, and works quickly.
Any image editing software should be able to do it. Just "Save As" .bmp