My tests show that in Outlook 2013, unlike earlier Outlook versions, setting line-height to 0 will cause the container to be smaller than the contained image. The result is that the top of the contained image is cropped. The only way I could fix this was to set line-height to a value other than 0. I used line-height: 17px and the images displayed properly. It might work with any other value than 0 -- I didn't test values smaller than 17px.
Setting display: block did not fix this. Since this ONLY is a problem with Outlook 2013 and does not affect any earlier Outlook versions or other mobile or desktop email clients, it's safe to assume it's an Outlook 2013 bug and that a fix from Microsoft isn't to be expected. So if anyone can confirm my findings, it'd be a good idea to just integrate this workaround into your already oversized palette of workarounds to accommodate Microsoft's poor HTML email support.