Design Statement:

I am mostly trying to explore the illusion of depth. I wanted to loosely mimic linear perspective by having drawing units repeat while changing exponentially in size and position. I also wanted to see the effect of having two aspects of the image give this sense of depth in different directions, i.e., of having two vanishing points compete for the viewer's assent. The rectangles are meant to be like windows that each offer a glimpse of a different part of an image behind the stings and circles and thereby preclude any real commitment to the illusion of depth they create. I intend for them to have an effect similar to that of the scene in Time Bandits when the invisible barrier shatters to reveal the Evil Genius's lair.

Technique Statement:

My techniques were much more imperative than functional.
For the overlapping ellipses, I wrote a procedure complementing-ellipses! that makes 8 calls to image-select-ellipse while adjusting the size and position of each so that the top and left edges stayed the same while the diameter was .75 that of the ellipse before it. I passed complementing-ellipses! a color that varied based on n by assigning in the image-series procedure itself a host of RGB colors to modulos between 0 and 18. I had each ellipse complement the last by entering (rgb-complement color) as the argument to (context-set-fgcolor) for every other ellipse. 
For the string-art spiral, I did not tamper with image-string-art! itself, but wrote another procedure that called it a predetermined number of times (7) with arguments that adjusted the size and orientation of each arch so as to form a spiral. Here I allowed for slight variation in the ratio by which size decreased, so that for each modulo between 0 and 12, the position/size of the arches is a bit different. I set the string color by doing an rgb-phaseshift of the color defined in the let statement of image-series and passed to complementing-ellipses! to ensure that the strings were different from both ellipse colors.
For the square-windows! procedure, my approach was again to call another procedure, here image-compute-pixels!, a predetermined number of times (3), and to vary its output by designating based on n (and using modulo) 1 of 7 colors for the image-behind-the-image to fade to, left to right, from black.