The letter K is a little out of date, and there is probably a better choice out there for Y, but here is a science-based alphabet poster.
A Science Alphabet
Scientific Gems
Facts, ideas, and images from the shoreline of science.
The letter K is a little out of date, and there is probably a better choice out there for Y, but here is a science-based alphabet poster.
Scientific Gems 
The obvious improvement for K is Kelvin. Or a K-type star.
I notice that you are interpreting Science as the beta sciences and not alfa or gamma sciences.
If you would allow for the Social Sciences the Y for Yerkes-Dodson Law or the Y-generation cold be options.
I thought of “K is for Kelvin,” but an obvious illustration didn’t spring to mind (and I’ve already got “thermometer”). K-type star might work.
And I was using “science” to cover physics, chemistry, geology, life sciences, etc. I have seen various formulations of “alpha sciences” and “gamma sciences” (there doesn’t seem to be a 100% agreed definition). While they are certainly within scope of the blog, I would see them as deserving their own posters.