Add Grid Image - Wordpress Tempalate

1) Hi I need to add a fixed image in one corner, in the shape of a cross or plus sign, to a wordpress template that displays a grid on the home page. The template is similar to the life and times website. This background image that is part of the grid is static and never changes, however the other images around it change. There are a total of 6 images across and 3 rows down. It looks like it can be accomplished by making an adjustment to the css and the main php file for the grid and maybe one of the lib files. 2) I also would like to change the css and the php files so that when you hover over each image on the grid, the color changes dependent upon the category that it is in. I know this is a simple fix, but I dont have time to play around with any further.