Hi!
As ThermioN said, it is related to the graphic driver; mine is AMD which strech all images into 16:9.
However, you can do some work arounds into DosBox.conf file in order to strech the image. So seek for the game specific file, open it with a text editior and seek for the
SDL section, and modify this values:
[sdl] fullresolution=desktop
output= surface | overlay | opengl | openglnb | ddraw
(I recommend
opengl or
overlay, but is know that Windows 8 have some compatibilities issues with OpenGL so you may try the option that suit the best for you.
A better description for those functions is explained here)
Also, according to the game, you may want to correct the aspect ratio, so seek this time for the
render section:
[render] aspect=true
More info and how to modify the DosBos conf file can be found here. Good luck!