I am hosting a Jekyll Blog on Github and write my posts with Markdown. When I am adding images, I do it the following way:

*image_caption*
This would generate the following HTML:
image_caption
Then in your CSS you can style it using the following selector without interfering with other em
tags on the page:
img + em { }
Note that you must not have a blank line between the image and the caption because that would instead generate:
image_caption
You can also use whatever tag you want other than em
. Just make sure there is a tag, otherwise you won't be able to style it.