Blitting Offscreen

I know that this is a silly question, but I have to ask it. I have been having problems blitting some 8Bit offscreen surfaces into the primary or back surface. This is the problem: When I scan 8Bits bitmaps buffers into the offscreen surface, and then I blit the offscreen into the Primary of Back surface, the image will be mapped correctly only if the offscreen and the image had specific dimensions. For example, if I have a bitmap image that is 20x37 pixels, I will create an offscreen that is 20X37 pixels. Then i will blit the offscreen, and in this case the image will be mapped correctly, and it will look fine. However, if I have a bitmap image that is 72X80 pixels, It is for sure that I will create an ofscreen that is 72X80 pixels, but in this case the image will be mapped incorrectly and it will be impossible to tell what it is. Can an image stored in a offscreen be distortionated by LPDIRECTDRAWSURFACE7->Blit() function when the offscreen doesn''t have specific dimensions? thanks for your help guys! ASW