For my work the most useful sites for 19^th-century art are the UK sites of the Royal Geographical Society, the British Museum, the Victoria and Albert Museum, the National Gallery, the National Portrait Gallery, and the US sites of the National Gallery and the Delaware Art Museum. These all offer images for scholars for free or for very nominal fees and since my needs include photographs, as well as paintings and fashion,  these sites offer many important images I can use. To create a list of these it might be useful to create in a file or perhaps on Zotero, a kind of bibliography of sites noting the most important content each site offers.