Here's a way to persuade SellerDeck to put the first item (like the Nivo Slideshow) in Brochure Fragment List full width with 2 or more columns below:
Go to Design / Library / Layouts / Fragment Lists.
Right-click Standard Brochure Fragment List and choose Copy.
Rename that copy to be Single First Item Then 2 Across Brochure Fragment List.
Double-click that layout to edit it.
Click “Click here to edit list layout settings” / Edit Rows And Columns / First/Middle/Last.
In First Row:
No of Items: 1
Before Row: <tr>
First Item / Before: <td colspan="2">
First Item / After: </td>
After Row: </tr>
OK out.
Now, in your home page / Layout / Fragment / Brochure Fragment List Layout, choose Single First Item Then 2 Across Brochure Fragment List.
The above is for a single item followed by 2 across.
For 3 across, follow the same instructions but put a “3” wherever a “2” is. Similar for 4, etc.