I am unsure about your question but my adivse would be:
Do not use TPaintBox or TImage to draw something at all.
Create a TBitmap or BGRA or Bitmap32 etc...
Whenever its needed, assign the "Bitmap" to an TImage.
The Bitmap can have any size while the TImage, as usual for an visual component, have its limits.
I hope I've answered your question?!