Picture in the Specifications section shows sizes of DIMM modules.
The picture is out-of-date - currently LS2088ARDB-PC is equipped with three 16GB DIMM modules which gives 32GB of main memory (third DIMM is intended for use by DPAA2).
For maximum supported memory sizes please refer to the QorIQ LS2088A Reference Manual, 12.6.1.1 Supported DDR SDRAM Organizations.