Hi,
I have made some investigations...
The image is resized inside the button to keep at least a padding of 6 pixels all around and to keep the aspect ratio.
A single unit button has a size of 47 pixels (all borders included). There is a gap of 8 pixels between the buttons.
Here is a table with the sizes.

- Daslight-ShowImageSize.jpg (54.33 KiB) Viewed 6508 times
So, if you need to see a true rectangular image for a button of 2 units wide x 1 unit high, you have to use an image of 90 x 35 pixels (or larger with the same width/height ratio).
Hope this helps.