To simply resize an image (manually) already in the media archive:
- Open it in the built-in Image Editor (in the Media perspective).
- Image / Scale image... (or click the Scale button on the toolbar - 2nd from the left)
You can also crop the image in the same Image Editor:
- Click the top button ("Select") on the floating bar. (The crop button on the toolbar will get enabled.)
- Use the cursor to select an area on the image.
- Click the crop button on the toolbar (3rd from the left)