Hopefully there IS some nicely named images...if not, you could just use sigil's renaming capability to give them good names...just right click on the image (or select multiple images) in the Book Browser/Images folder, then select 'rename'.
Expanding on the example above:
<img alt="BigA" src="../Images/BigA.jpg"/><p>ardvark</p>
Code:
search: <img alt=".*?" src="../Images/Big(.)\.jpg"/><p>(.*?)</p>
replace: <p class="firstpara">\1\2</p>
and include whatever styling you wish for the first paragraph of a chapter:
p.firstpara {yada yada}