ImageMagick

Sometimes its a pain when you want to modify, resize multiple files at once in Linux. Fortunately ImageMagick comes with a handy command called convert that makes the work simpler Install ImageMagick in Ubuntu
sudo apt-get install imagemagick
Batch Process set of files in a folder and resize them to 1024X768
for file in *.jpg; do convert "$file" -resize 1024x768 "$file"; done
You may also try an app called as phatch for GUI related batch processing of images. Snippet to bulk rename files by replacing strings in filename