Bootstrap News Help

I want to improve the design of my website. Rather than altering the image myself via Gimp every time I want to post, one feature I want to add is that the main category displays in the corner of the image. I basically want something similar to what the CoverNews theme provides, but I want to remain on the Bootstrap News theme.

How would I manage something like this, is there a plugin or is this only able to be done via HTML / CSS? What would I search for to learn how to add this feature if it is that way?

The page I need help with: [log in to see the link]