Many old master paintings of lutes can be found by putting the name of a painter in a general search engine plus the word lute.
The distinction between online and real picture galleries is to some extent blurred as some 'real' galleries make their artworks available via consortia and collective online virtual museums.
ONLINE-ONLY MUSIC PICTURE GALLERIES SEARCHABLE FOR LUTES
Wayne Cripps' site, www.cs.dartmouth.edu/~wbc/icon/icon.html
Cuerdas Pulsadas site: www.cuerdaspulsadas.es/timeline/
Historic paintings illustrating technique on Jean-Marie Poirier's site: le.luth.free.fr/
Thomas Schall's site, www.lautenist.de/LutePictures.htm
Igor Varfolomeev’s lute and guitar site, www.lute.ru/home_eng.htm
Lute art links in the Lute Society of America site: www.cs.dartmouth.edu/~lsa/links/ArtLinks.html
A Lute Society searchable database of lute pictures is in preparation.
ONLINE-ONLY GENERAL PICTURE GALLERIES SEARCHABLE FOR LUTES
The web gallery of Art (93 hits for ‘lute') www.wga.hu/
ArtCyclopedia (searchable for ‘lute’ in title) www.artcyclopedia.com/
The Utrecht Emblem Project: emblems.let.uu.nl/
Glasgow University Emblem project: www.emblems.arts.gla.ac.uk/
Europeana is a virtual repository of artworks, literature, cultural objects, relics, and musical recordings/writings from over 2000 European institutions www.europeana.eu/portal/
Khan Academy's smARThistory is a multimedia resource with videos, audio guides, mobile applications and commentary from art historians www.khanacademy.org/humanities/art-history-basics/beginners-art-history/a/cave-painting-contemporary-art-and-everything-in-between
LISTS OF LUTES IN ART
List compiled by Max Sauerlandt: www.cs.dartmouth.edu/~wbc/Sauerlandt/Sauerlandt.html
Mary Rasmussen’s Musical Iconography, alphabetical by instrument: www.unh.edu/Archives/music/igref.htm