I want the bottom image of the header to be flush with the Category bar of the board but I can't find the padding that controls it. The distance is 3px but I can't find the line of code. Maybe the bottom padding of the Header is 1px and the top padding of the Category bar is 2px or something like that?
This actually has to do with how the browser treats spacing following a line of inline elements (such as text and images) rather than the margins of the elements around your images. I can actually demonstrate this within a post.
Here's two images on top of one another. You'll notice there's a gap between them because of how lines are spaced.
Now watch what happens when I place them inside of an element with a font size of 0.
The gap disappears because when the font size is 0 the space between lines is also 0. With that in mind the easiest solution is to surround your image with a div element that has a font size of 0 which will tell the browser not to add a margin below the line your image is contained on.