We love this online photo collection of Budapest and other places in Hungary from 1900 through about 1990. Fortepan has been online since 2010 and has grown to about 80 thousand images under the direction and creative genius of volunteer editors and helpers. All of the images are licensed under Creative Commons CC-BY-SA-3.0 and can be freely used; you just need to provide proper attribution if you publish one of them.
Try a full-text search for “nador utca 9 zrinyi utca” and see what this CEU building looked like in 1950!