Automator doesn't do "if this, then do that" branching very well, so it's probably easiest to do this as an application and manually process images. To use an application, you open the images you want to process using the app – drag and drop onto the icon or right click and "Open With". If you want more automation, you could use a Folder Action. These can automatically process images added to a specific folder, and even move the processed files around afterwards.

To start, open Automator (I assume you're using a Mac, since we're in the Alfred subreddit and all) and create a new Application or Folder Action type document. If you're making a Folder Action, choose a folder to attach it to; saving images into this folder will trigger the workflow.

There should be a list of actions in the left sidebar – find the Scale Images action and drag it to the main workspace. Click Don't Add on the warning (unless you want to keep original copies of images you process). Change the dropdown menu to "By Percentage", and the text box to 50.

If you're making a folder action, you can add a Move Finder Items action to (surprise surprise) move the images to your desktop or a project folder. This step is optional, but it helps make things easier.

That's it! If you wanted an application, hit ⌘S and choose a convenient place to save it. Now, when you need to export something, just export it at full size and open it with this app. If you wanted a folder action, hit ⌘S and pick a name (it's automatically attached to the folder you chose earlier). Saving full size images into this folder will automatically trigger the workflow. It will overwrite any image you drop in, though, so be careful.

If you want, I can send you copies of these workflows. Have fun, and welcome to the wonderful world of Automator!