Adding (custom) thumbnails to individual posts

Hey,

I have a question about adding thumbnails to individual posts. First look at my uploaded image.

Example

I want to do a couple of things:
1. I want to add an image to my post which automatically will be resized to a certain size (in this case 100x100px).
2. I (always) want to overlay this thumbnail with a semi-transparent PNG-image given it the look in my example.
3. It has to be positioned top-left in my post.
4. (For my client) it has to be easy to do.

I can manage a part of it hard-coded by using two divs (one for the actual image and one for the overlay image) and positioning them absolute and playing around with z-indexes. But I cannot figure it out using WordPress (as a CMS).

So, what is the best solution, and what do I have to look for?