I figured out what the problem was. I'm not sure what the purpose of it is, but within template.css, there appears the following code:
.pages_portfolio img {
margin-bottom:-7px;
}
I added to the custom.css the following override:
.pages_portfolio img {
margin-bottom:0px;
}
The problem is now fixed, but I was curious as to the intent of the original code? Why add a negative margin when there is already a 5px margin and padding already added to the div housing the image in the first place?