Issues with Images within a table

I am not sure this is the correct location to post a technical question but hope that someone out there may have run into this situation. I have inherited a website for which I manage content; the owners of the site would like to have a "cluster" of pictures ... one larger picture on the Left with three smaller pictures to it's right forming a large 'rectangle' with white spaces dividing the 4 images. The easiest way I could find to make this happen was to create a 2-column, 3-row table with the 3 cells in the first column being merged. I can set the cell spacing to the desired thickness of the white boarders but am having difficulty with the image dimensions. Here is what I had to date:

- Cell spacing is set to 4; cell padding is set to 0.
- Column 1 is 450 pixels wide; the one cell is 3*100 = 100 + (2*4) = 308 pixels tall (to try and account for the cell spacing between rows 1/2 and 2/3 in the 2nd column.
- Column 2 is 150 pixels wide; each row is 100 pixels tall.

When I try to insert the 450x308 and 150x100 pixel images they don't quite align all the way around. It is almost as if I have not accounted for the cell spacing properly. Is my math off? I understand that it would likely be easier to amalgamate the images into 1 larger image and post it but when they want the images changed I want to simply be able to replace the 1 or 2 images without composing a whole new picture.

Any thoughts or ideas would be appreciated.