Unfortunately there is no standalone application to publish Hammer created maps. However you can still publish your Hammer maps to the Workshop via a workaround that uses the ingame map editor.

Creating the custom thumbnail can be a bit tricky because Portal doesn't seem to like some jpegs. For me it wouldn't accept .jpg files that were made in Adobe Photoshop or Microsoft Paint.

If you have a .jpg that you would like to use, you can test if it will work with the following two steps
1. Start "steamapps\common\portal 2\bin\p2map_publish.exe", click `Add` and browse for your image.
2. If your image shows up (and it is a .jpg) you can skip to step 6. If you get the error "Unable to parse preview image!" you have to do the following steps first. If you are using any other file type than .jpg you also have to do the following steps.

These steps are required if you have a non working .jpg or any other file type image
3. Convert your image to .png using any image editing software if it isn't already.
4. Start "steamapps\common\portal 2\bin\p2map_publish.exe", click `Add` and browse for your .png image and it should show up as the preview image. Call the map "Test", add a random .bsp file and click `Publish`. Don't forget to set it to hidden in the Workshop or remove it after following this guide.
5. Once it is published, select it again and choose `Edit`. Now close p2map_publish and navigate to the folder "steamapps\common\portal 2\bin\p2map_publish". Here you should find your thumbnail in the proper jpg format.

Follow these steps if you already have a proper jpg or you have created it in the previous steps
6. Copy your .jpg image (it should be 555*312) to "steamapps\common\Aperture Tag\portal2\puzzles\<your_id>" and name it "untitled.jpg"
7. Set the file attribute to read-only. Don't forget to remove the file after following this guide.

1. Start Aperture Tag and create a new test chamber.
2. Choose File -> Publish and let it compile until you see the "Publish to Workshop" screen. If you added a custom image you should now see it here.
3. Alt-tab out of the game and navigate to the folder "steamapps\common\Aperture Tag\sdk_content\maps". Search for the .bsp and .vmf files that were just compiled by looking at the modified date. It will be called something like "1407329816".
4. Open up Hammer and load the map you want to publish, save and overwrite the .vmf file from the previous step and recompile your map.
5. Alt-tab back to the game and fill in the name and description for you map and click `Publish Test Chamber`.
6. The map you created in Hammer should now show up in the workshop with the custom thumbnail.