I have set up a couple of custom actions in Thunar (my file manager) to reduce files.
Thunar has an option "Edit -> custom actions" to add options to the context menu.
You can then define commands to be run on selected files / folders etc...
mogrify -resize 1000x1000 %f
would take the selected file and reduce it to 1000px. %f stands for the selected file, but multiselections etc. are also possible with other place holders. You can even call scripts and hand the files over as arguments to them. You get the picture I guess.
And there is also an option to make Thunar understand that you only need this option for image files.
I have added several options such as percentage sizes, absolute sizes 1000 and 2000 px edge length etc... I find that a lot more convenient than having to open a gui tool everytime I want to email a picture.
Other filebrowser also support custom actions, but you'd have to research how the configuration works...