Nautilus Image Converteris a plugin or extension for Ubuntu‘s file manager that lets you quickly and easily resize and rotate image files. Once installed, all you have to do is right-click a picture file, or a group of them, and choose either “Resize Images…” or “Rotate Images…“.
In the Resize Images dialogue you can choose from predefined sizes via “Select a size:” (96×96 & 128×128 for thumbnails or avatars, as well as wallpaper dimensions for older 4:3 screen resolutions 640×480, 800×600, 1024×768 & 1280×960), “Scale:” it to a certain percentage, or choose your own “Custom size:“.
You will note that for “Append” the default is to save these altered images as copies with .resized added to the file name. You can choose “Resize in place” instead, but note that this overwrites the original, so use with caution.
With the Rotate Images dialogue, you can choose from 3 predefined angles via “Select an angle:“, being 90° clockwise, 90° counter-clockwise and 180°. If you need a bit more precision than that, just define your own angle with “Custom angle:“.
The image converter options will be available in the context menu after a reboot, or you can log out and back in again if you want the changes to take effect immediately.