Many customers have reported after uploading a workbook to the PowerPivot Gallery, the thumbnails are not rendering:

Snapshots will not work if you have chosen to open the workbooks in the Client application. To remedy this, follow the below instructions:

To set the default open behavior for site collections

1. In the SharePoint site collection, click Site Actions, and then click Site Settings.
2. On the Site Settings page, under Site Collection Administration, click Site Collection Features.
3. On the Features page, for the Open Documents in Client Applications by Default feature, click Activate (OpenInClient Feature is enabled) to open documents in the client application. Click Deactivate (OpenInClient Feature is disabled) to open documents in the browser.

To set the default open behavior for a document library by using the document library settings page

1. On the <document library>: All Documents page, under Library Tools, click Library.
2. In the library toolbar, click Library Settings.
3. On the Document Library Settings page, click Advanced Settings.
4. On the Advanced Settings page, in Opening Document in the Browser, select one of the following options:
   Open in the client application When a user clicks on a document in this library, the document will open in the corresponding client application (if available).
5. Open in the browser When a user clicks on a document in this library, the document will open in the Web browser Web app for that document type. When the document is opened in the Web app, the user can then decide to open the document in the client application
6. Use the server default When a user clicks on a document in this library, the document will open by using the default open behavior specified for the server on which SharePoint 2010 Products is installed.

If the above do not work:

You need to invoke the Getshapshot.exe (which is located here: C:\inetpub\wwwroot\wss\VirtualDirectories\80\bin>GetSnapshot.exe)

        *Note: Browse to C:\inetpub\wwwroot\wss\VirtualDirectories\80\bin>GetSnapshot.exe and make sure it is 50/56kb.  I have seen instances where is is corrupt 0kb and it needs to be replaced.

To invoke the Getsnapshot.exe (re-title a workbook) via:

PowerPivot Gallery > Select a Report > Documents > Edit Properties > Title (give the workbook a title):    

If these steps do not remedy this issue, please move onto my next article:

Invoking and Analyzing the Getsnapshot.exe
http://blogs.technet.com/b/excel_services__powerpivot_for_sharepoint_support_blog/archive/2012/12/06/invoking-and-analyzing-the-getsnapshot-exe.aspx