If you have a workbook with a Power View worksheet that was created in Excel, and you upload that workbook to the PowerPivot Gallery in SharePoint, you may receive a red X instead of a thumbnail because of a failure with the snapshot generation:
This is expected behavior as the snapshot process does not support embedded Power View reports in workbooks.
One workaround is to only publish all worksheets except for the Power View worksheets when uploading the workbook to the PowerPivot Gallery. Click on Save As in Excel when saving to SharePoint and you will get a Browser View Options button in the Save As dialog box:
Click on Browser View Options, and on the Show tab, select the dropdown menu and change Entire Workbook to Sheets. This will allow you to uncheck the Power View worksheet:
Click OK and continue to save the workbook. The Power View worksheet will not be displayed in the PowerPivot Gallery thumbnails or in Excel Services when you open the workbook, but it will still be visible when you open the workbook in Excel client. This allows the thumbnails for the rest of the worksheets to be generated successfully in the PowerPivot Gallery, but you will lose the ability to view the Power View report in the browser. As you can see below, the third worksheet, which is the Power View worksheet, is not shown in the workbook thumbnail, and is not part of the worksheet thumbnails:
Another option is to place the Power View report as the last worksheet in the workbook, and then display the first worksheet before saving the file. This will allow snapshot generation of all worksheets before the Power View worksheet as the worksheets are enumerated in order during the snapshot process. As you can see below, the workbook snapshot shows that there are 3 worksheets, but only thumbnails are only generated for the first 2 worksheets as the third one is a Power View worksheet:
The advantage of this option is that the Power View worksheet is published to SharePoint, so even though the thumbnail is not generated, when you open the workbook in Excel Services, the third worksheet is visible and the Power View report is rendered.
In order to view a Power View report thumbnail, you will need to use Power View for SharePoint to create the Power View report instead of creating a Power View worksheet in Excel. To do this, upload the PowerPivot workbook to the PowerPivot Gallery. Next to the thumbnail for the workbook, you have a few icons, one of which allows you to create a standalone Power View report in RDLX format based on the PowerPivot workbook. When you hover over the button it says Create Power View Report:
After building the report and saving it to the PowerPivot Gallery, a thumbnail will be generated because in this case, Power View generates the thumbnail instead of PowerPivot using a different snapshot generation process:
Please note that the Power View feature for SharePoint is only available with SQL Server 2012 SP1 and above as part of the Reporting Services add-in for SharePoint.