For Dialog-based skin the logo image is stored in logo.bmp file (located in Bin directory).
Its format is BMP. You can replace it with your own logo with the same format and approximately the same size. Size is not very strict (if the size of image is too big, it will be cut off).
The program draws the logo with a transparent background. In order to make this feature to work with your new logo, make sure that the background of your file has a clear white color (RGB 255, 255, 255). In other case the transparency will not be applied.