oneruffryder on "How to add icon over thumb"

Hi all, i m not so good with php, but i wanted to do this, and i know its possible, but i dont know why. I m looking at http://codex.wordpress.org/Function_Reference/in_category but it doesnt make to much sense.

Basically what i want to do, for an example on my homepage, category page and similar post are shown with title and a thumb.

And for this thumbs i want to add function like this

if post/thumb is inside category video -> add video icon above it. I guess with css i would need to position it for an example at the bottom right corner.

Any ideas how to this