Contentful provides a basic mechanism for resizing and cropping images, which is sufficient in most cases. However, if you require additional flexibility, you will need to edit your image outside of Contentful and upload it again.
This app integrates with the Filerobot Image Editor (https://github.com/scaleflex/filerobot-image-editor), enabling you to edit images directly from the Entry editor. Ideally the Image Editor should be located inside the Asset editor, but Contentful Apps are not allowed there.
Download and installation instructions on github https://github.com/pauloamgomes/contentful-image-editor-app