Here is the fastest way to resize a picture in linux through command line.
Download imagemagick and install it using the ./configure, make, make install or if your using Ubuntu you could sudo apt-get install imagemagick
Open terminal (Alt+F2 gnome-terminal)
Go to the director of your picture, eg desktop
cd ~/Desktop
Convert your picture
convert -resize 100x100! myPic.jpg myPic.png
Explanation:
convert is the program you need
-resize is the function you need for the resizing
100×100! – destination width and height, add ! to force it to 100×100, remove ! and it will respect the ratio of the picture
myPic.jpg is the source file or the file you want to resize
myPic.png is the destination file after the resize is done
Hope it helps