Legionary shields in the Second Punic War

[tiberius aemilius naso]

As Sean says, there are not many artifacts left. Just some umbos, and none of them from the 3rd C BC, the earliest umbos we can consider Roman without doubt are from the 2nd c BC. But that isn't what you are asking for! Concerning decoration, we just don't know. We know nothing. So you can just make it up, but keeping it plausible. How? IMHO the best (and coolest and probably more accurate) way would be to use motifs from Republican coins from the 3rd C BC. Victories, eagles, the Dioskuri, horses, Mars... look for the user Andrew McCabe (Ahala) in Flickr, you have all the coins there!