The Memory Album is a feature found under the More tab that allows you to review any reward photos that you have unlocked this play through. Currently, there are eight pictures available per girl, four rewarded as you advance your relationship with them, and four more unlocked the first time you take that girl on each of the different dates.
A Halloween-themed set of date pictures was added with v.0.78 on Steam on October 24, 2016; the pictures returned back to normal per November 1st as the developers stated that they should.
On December 7, 2016, Crush Crush came with v.0.86 introducing two new girls; Karma and Sutra. The Beach date and Sightseeing date pictures for all girls (not just Luna and Eva) showed the Dark Portal in the background, once it has been opened by unlocking Luna, as of this version too.
On December 24, 2016, a Winter-themed set (aka Xmas Set) has been added with v.0.94 on all platforms (Steam, Kongregate and Nutaku) along with Holiday Outfits for all Girls.
On February 22, 2017, The Dark One and Q-Piddy have been added. If you complete the Ending Scene (either path), the Dark Portal will fade from sight in the Memory Album for all girls.
Beyond v. 0.155, The Memory Picture will remains regardless of Soft Reset(With Steam Cloud sync) with addition of Pin-up Pictures if you completed the Event made.
Those pictures will only remain in your Memory Album if you Completed the Event.