eBay Hosted pictures not showing the 800x800 image size

Summary 

You must make sure you upload the image and list the item correctly to enable the larger images on your listings. Upload the image with PictureSet as "Supersize" and list the item with PhotoDisplay either as "PicturePack" or "SuperSize".
 

First step is to upload your pictures correctly. They must be uploaded with the PictureSet set to Supersize"

<?xml version="1.0" encoding="utf-8" ?> 
<UploadSiteHostedPicturesRequest xmlns="urn:ebay:apis:eBLBaseComponents">
  <ExternalPictureURL>http://YourImageURL</ExternalPictureURL>
  <PictureSet>Supersize</PictureSet> 
</UploadSiteHostedPicturesRequest>

This has results similar to:

<SiteHostedPictureDetails>
  <PictureSet>Standard</PictureSet>
  <PictureFormat>JPG</PictureFormat>
  <FullURL>http://i1.sandbox.ebayimg.com/03/i/00/24/15/13_1.JPG?set_id=8800005007</FullURL>
  <BaseURL>http://i1.sandbox.ebayimg.com/03/i/00/24/15/13_</BaseURL>
  <PictureSetMember>
    <MemberURL>http://i1.sandbox.ebayimg.com/03/i/00/24/15/13_0.JPG</MemberURL>
    <PictureHeight>96</PictureHeight>
    <PictureWidth>74</PictureWidth>
  </PictureSetMember>
  <!-- more PictureSetMember containers -->
  <ExternalPictureURL>http://www.mysite.com/55998_55998_441_9.jpg</ExternalPictureURL>
  <UseByDate>2010-01-11T00:57:43.710Z</UseByDate>
</SiteHostedPictureDetails>

The second step is to list your item correctly. When using one of the AddItem family of calls, include this PictureDetails container (Note the use of the FullURL):