So, for a code to 'scale and proportionately resize an image to the sketch window' here's one way it could be done using a function.  In this code, I already have an image loaded as 'bgimg' and I create a new temporary image (tmpimg) because I also need to preserve and use the scale of the original bgimg, and then I scale the tmp image and draw it into a buffered PGmage (lines 13-18).  You could pass variables to the function such as background color, to make it more extend able.

Line 4 checks to see if the image is bigger than the sketch window.
Line 5 checks if the image is wider than it is taller; if so, then it scales by image width.
Line's 8 and 9 check if the image is taller than wider and scales by image height if so.

Lines 13 -18 create a buffered image with black background the size of the sketch and places the resized tmpimg in the center.

After the function is called, you can then draw the scaled and framed image with the image command, e.g. image(sbgimg,0,0), use it as a background (background(sbgimage);) or pass it off as an image to other functions or classes for further processing (myWeirdThings = new WeirdThings(sbgimg,99);).

I decided I didn't want to rescale images that are smaller than the sketch window to fit in the whole sketch window so this function doesn't do that; but it would be easy to do with the code below and some modifications.

with declared PImage BKimg; PGraphics sbgimg; PImage tmpimg;

1. public void centerit() {
2.   float minborder = 10;
3.   tmpimg = bgimg;
4.  if (bgimg.width >= width || bgimg.height >= height) {
5.   if (bgimg.width >= bgimg.height) {
6.     tmpimg.resize(dwidth-10,0);
7.   }
8.   if (bgimg.width < bgimg.height) {
9.     tmpimg.resize(0,dheight-10);
10.   }
11.  }
12.  
13.   sbgimg = createGraphics(dwidth, dheight, P2D);
14.   sbgimg.beginDraw();
15.   sbgimg.background(0);
16.   sbgimg.imageMode(CENTER);
17.   sbgimg.image(tmpimg,width/2,height/2);
18.   sbgimg.endDraw();
19. }