Here's a way to persuade SellerDeck to put the first item in a Brochure Fragment List (like the Nivo Slideshow) full width, with multiple columns below for subsequent items:
Go to Design / Library / Layouts / Fragment Lists.
Right-click Standard Brochure Fragment List and choose Copy.
Rename that copy to be Single First Item 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="<actinic:variable name="FragmentColumnCount" />">
--First Item / After: </td>
--After Row: </tr>
OK out.
Now, in your home page / Layout / Fragment / Brochure Fragment List Layout, choose Single First Item Brochure Fragment List.