Programmatically change image resolution
If you have generated your image 7016 x 4961 px, it is already A4 at 600 dpi. So you don't need to resize it, you just have to set resolution information in file.
You can do it with PIL:
from PIL import Imageim = Image.open("test.png")im.save("test-600.png", dpi=(600,600))
This code will resize a PNG image into 7016x4961 with PIL:
size = 7016, 4961im = Image.open("my_image.png")im_resized = im.resize(size, Image.ANTIALIAS)im_resized.save("my_image_resized.png", "PNG")Perhaps a better approach would be to make your canvas x times bigger prior to printing, where x is a factor you have to figure out (7016x4961 in size for this particular image).