See this answer revision: the images are floating above the heading's text-bottom line.
This is caused by images having a margin-bottom: 0.5em
. This declaration isn't needed inside headings because those headings already have a margin to space them away from surrounding text.
Instead images inside headings should have margin-bottom: 0
which will let them sit at text-bottom.
I suggest adding the following CSS:
.post-text h1 img,
.post-text h2 img,
.post-text h3 img {
margin-bottom: 0;
}