The splash screen is the first impression that new users have from your application - I would not make any compromise: display the unscaled image; if it is too large or too small find another one.
To display the unscaled image set the AutoSize of the TImage (which contains the splash image) to true, do not set its Stretch or Proportional to true. And also set the AutoSize of the splash form to true. This way you always have the best quality, and all sizes adjust themselves so that nothing is clipped or left uncovered.