Can be done by using ImageMagick.This is a software suite to create, edit, compose, or convert bitmap images. It can read and write images in a variety of formats over 100. Using ImageMagick let you to resize, flip, mirror, rotate, distort, shear and transform images, adjust image colors, apply various special effects, or draw text, lines, polygons, ellipses and Bézier curves.
Install the software under Fedora:
# yum install ImageMagick
Convert an image from one format to another. In this case jpeg.
$ convert *.png *.jpg
$ mogrify -format png *.jpg
Read more about this software here.