PictureResize is a script which, when installed, will create an icon on your Linux desktop.
Installing the script:
Dragging and dropping any image on the PictureResize icon will create a resized copy of the image in the same location as the original image. For example the file DSC02422.JPG dropped on the icon will result in the creation of another file DSC02422-640.JPG, which will have the image with its width resized to 640 pixels.
Resized images are easier to upload on Facebook and similar sites, because in reducing the dimensions of the image, the actual size of the file also drastically reduced, making the upload much faster.
You can even drag and drop a whole folder on the icon, and all the images in it will be resized.
If you want to change the default width of the resized images to something other than 640 pixels, you can right-click on the PictureResize icon, and click on "properties". There will be a "command" displayed, apart from other things. The command will have an option "-s 640". You can just change it to whatever size you want, e.g., "-s 1200", and click on the close button.