It is rather easy to do basic picture manipulations using the shell. The tool imagemagick is extremely powerful and is part of the apt system, so it is easy to install with
[shell]
sudo apt-get install imagemagick
[/shell]

In order to resize a jpg picture (called here: pictureIn.jpg) then to a certain width (e.g. 2000px) and maintain the width/height ratio it is enough to type

[shell]
convert pictureIn.jpg -resize 2000 pictureOut.jpg
[/shell]

At the same time it would be possible to reduce the quality of the resulting picture, to further reduce the file size. For that the command would be

[shell]
convert pictureIn.jpg -resize 2000 -quality 90 pictureOut.jpg
[/shell]