I was wondering if there was a repository of flow visualization results somewhere online, preferably of wing sections categorised in some manner.
The closest you will find to a comprehensive, freely accessible source of data on this is the NASA Technical Reports Server. It is a search tool that gives you access to many NACA reports on various airfoils.
You can check the ERCOFTAC Classic database collection at: http://cfd.mace.manchester.ac.uk/ercoftac/index.html
It is a comprehensive database used by fluid-dynamicist for model validation, and it contains many flow visualizations including a couple of wing test cases. Experimental data are free to download.