how do i convert pdf to jpg in automator?

Automator has a "Render PDF Pages to Images" action.

There's a wizard that pops up when you run Automator asking you to choose a template. The second template is "Application":

Applications are self-running workflows. Any files or folders dropped onto an Application will be used as input to the workflow.

So:

1. Open Automator
2. Choose "Application"
3. Drag the "Render PDF Pages to Images" action onto the workflow
4. This action produces random filenames, so follow it with "Rename Finder Items" (choose "Make sequential" from the dropdown list
5. Then add a "Move Finder Items" to have the output files moved to the Desktop
6. Save your new application somewhere
7. Drag files onto it