css equivalent of first:child to target first <p> in div with heading and img preceding?

The most straightforward way to do it is to use CSS3's :first-of-type:

#feature > p:first-of-type {
    font-size: 22px;
}

But browser support for it is poor, so for your case you can try using this CSS2-only selector, assuming the first p is always followed by an h2 and an img (from your question title):

#feature > h2:first-child + img + p {
    font-size: 22px;
}

you can target the element above the p, then use "+" adjacent selector to select the p.
so in your case would be:

#feature img + p {
    font-size: 22px;
}

and it supports from IE7

Check this working code on Fiddle