xpath get element by index

In XPath index starts from 1 position, therefore

//div[@class="post-content"]//img[2]

should work correctly if you have to select each 2nd img in div[@class="post-content"].
If you want to select only 2nd img from all images that are in div[@class="post-content"], use:

(//div[@class="post-content"]//img)[2]

indexes in XPath are 1-based, not 0-based. Try

(//div[@class="post-content"]//img)[position()=2]