How to create an Automator service to convert images from one file type into another file type using the right-click contextual menu?
Here is an example Automator workflow to convert image files to a specific type:



When creating your workflow, don't forget to set it to a Service so that it can be accessed in the context menu.

The Copy Finder Items makes sure that the original files don't get replaces. You can change the save location to a variable if you wish, or just delete that action and let it replace the original file.

If you would like the option of which file type you would like, you can create multiple service workflows with different names (and different file formats for the Change Type of Images action).