Open the ePub is Sigil, switch to Code View and type ctrl-F
Find:
<img
Replace:
<img class="image"
Replace All
Fill in the appropriate css in the stylesheet.
The only possible issue is that Sigil will recreate the generated ToC, but as long as you've associated your Chapter headings with the appropriate h1, h2, h3 etc tags in the style Export Options that shouldn't cause a problem.