Every post requires an image that will be associated with it.  This images will appear on the home page for that post and in the different sections of the website.  This images should be representative of the particular post you are writing.  I usually try to choose my strongest image for that post since it will be seen the most.  It is also recommended to use a horizontal image because vertical images will be cropped unpredictably in the many different uses for the website.

1. Upload an image for use as the default image, if it has already been uploaded for use in the current post you do not have to upload it again.  Directions for uploading images can be found here.

2. While editing a post click the small square next to "Upload/Insert" (circled below)

3. Click on the "Media Library" tab at the top

4. Click "Show" next to the image you would like to make the default image.

5. You should see the image options like below, you have to highlight the entire "Link URL" if that field is blank click the "File URL" button at the bottom to show the file's URL then highlight and copy it. To copy press (Control+c) on windows or (Command+c) on Mac.

6. Click the little "x" at the top right of the image options window, all we needed to do was copy the file URL, we don't need to insert anything at this point.

7. Next place your cursor in the "Image" text field under "Aperture Custom Settings" and paste in the file URL we just copied in step 5.

8. Click the "Save" button, you may see a warning like below, click the "OK" button and you should see the image loaded in the "Aperture Custom Settings" like the diagram above.

That should do it for adding a default image to your post.  If you would like, you don't have to follow these exact steps the only thing you really need at the end is to place the "File URL" to an image in the "Aperture Custom Settings" section, if you have another way of doing that feel free to give it a shot.  If you find an easier way, please post it as a response here.