What you need to do is make a resolution grid. This will allow you to map out on your monitor exactly where you want you graphics to occur.
So...
Make a new Photoshop document. You want 640x288 at 72 DPI. (this will not be your layout so do not worry about the low res for now). Under view, click ruler. Under preferences, set your units to pixels.
Press - D (Sets colors to defaults)
Press Alt + Backspace (Fills the layer with the foreground color.)
Then make a new layer. (Ctrl + Shift + D)
Use the line tool (located under the text tool. You may need to click from the drop down set)
Make sure that the weighting is set to 1 px. Also make sure that the line tool is not set for paths. In the upper left hand side of the program there are 3 boxes when line tool is selected, make sure that the 3rd one (the 3rd from the left) is selected. This will make a rasterized line.
Press x (exchanges foreground for background)
White should now be the main color
Make a vertical line every 50 pixels
For an easier line click at the start location and press shift as you drag. This will lock the line vertical, horizontal, or 45 degree diag.
After you have all the 50's done you might want to add a few lines every 10 pixels for the first and last 50 pixels. =====
After you have your verts, then do the same but horizontally. Again making the first and last 50 pixels with 10 pixel increments.
Then get the text tool and mark the increments. don't make the font too small or it will be hard to see on the monitor, but don't make it so big that you can not tell which line goes to which number. You may need to stagger a text for the first and last 50 pixel sets (or you can just count the lines on the screen).
Once you have labeled the Hoz and vert lines save this as a PSD (DO NOT FLATTEN THIS FILE). You always want to keep PSDs editable. Once it is saved, click save as and name it as grid or something. Take the file and make a bogus layout for MAMEWah. Load it up.
Things to make sure of first is that 1. You have played some vert and hoz games and you are happy with the screen location adjustment. Then with the new mamewah layout, you can see exactly how the graphic will fall on the screen. Write down (do not try to remember) where the screen cuts off on the sides, top and bottom.
To make your layout, make a new document. 640x288. With the rulers on set up some guides by dragging from the ruler out onto the canvas. Place a guids for both sides, top, and bottom where the lines went off the screen in MAMEWah. Once you have your edges, then up the resolution. I would step up the resolution. 1920x864. This is 3 times bigger and you can work with more detail. When it gets squished down it will look better than if you started small. You want to increase your resolution BEFORE you start to work. You will notice that when you enlarge your image that the guides you set also stay in scale with the image. Cool eh'. Make sure that you have a background color or pattern or design that goes the full length of the 640x288, but keep your design work within the boundries you set prior to enlarging the image. Save often! Once you are finished, then save the file and then save as a jpeg. Once a jpeg then resize the image down to 640 x 288. Insert into MW and add your game list and snaps and play on!