Billboard Image Resizing

I have looked through the documentation and could not find any answers for my question.  There are no size guidelines for an image to be added to a billboard.  So when I add an image that I think is the correct size it either shrinks and or enlarges the image.  Also, if you resize the browser window the image actually shrinks and or expands.  Bottom line is that its impossible to get an image with Text to show correctly between a wide aspect monitor, a laptop monitor and an iPad.  Its either too big and the text is cut off, or to small and does not fill the entire space.  

Even if the image does not have text then changing aspect ratio's the image gets too large or small.  So what am I missing?  How can I fix this?

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