There are a whole bunch of people who regularly post great pictures from medieval manuscripts on Twitter. If you're on Twitter and interested, you might check out these accounts:
Among academics, I've already mentioned Erik Kwakkel (@erik_kwakkel), but also try Johan Oosterman (@JohanOosterman), Damien Kempf (@DamienKempf), Jonathan Hsy (@JonathanHsy), Robert Miller (@robmmiller), John Overholt (@john_overholt), and Peter Stokes (@pa_stokes).
Among thematic accounts, try Discarding Images (@discarding_imgs), Sexy Codicology (@SexyCodicology), and Manuscripts Online (@MedMSSO). Two fun accounts not strictly focused on manuscripts: Chaucer Doth Tweet (@LeVostreGC) tweets in the persona of a snarky Geoffrey Chaucer, while Medieval People of Color (@medievalpoc) focuses on images of people of color in works of European art history. (MedievalPoC is also on Tumblr.)
A number of libraries have great Twitter feeds where they post images from their own collections: the British Library (@BLMedieval); the Walters Art Museum in Baltimore (@MedievalMss); the Bibliothèque nationale de France (@GallicaBnF); the University of Nottingham (@mssUniNott); the US Library of Congress (@librarycongress [not strictly manuscripts]); the Newberry Library in Chicago (@NewberryLibrary); the Beinecke Library at Yale (@BeineckeLibrary); Houghton Library at Harvard (@HoughtonLib); the Huntington Library (@TheHuntington); the Getty Museum (@TheGetty); and the Vatican Library (@vaticanlibrary and @DigitaVaticana). The handle @manuscripts, sadly, got nabbed early on by the University of Cape Town, which posts virtually nothing medieval.
The hashtags #manuscripts and #paleography are pretty self-explanatory, but there's been a fair amount of interesting Twitter traffic recently under the the hashtag #DDMss (for Digging Deeper: Medieval Manuscripts), a MOOC currently being run by Stanford professor Elaine Treharne (@ETreharne) and Cambridge professor Orietta Da Rold (@orietta_darold).
I'll stop there, shall I?