Step by step, museums are putting their entire collection of paintings online. These archives make for a great source of images: consistent in style, without copyright issues (if you go back sufficiently far in time). You won't find stock images of smartphones though, but maybe that makes your presentation actually look better.
Here are some examples of well developed web sites:
There are still big differences in how advanced museum web sites are, but ultimately every museum will come to realise that access to their collections should not be restricted to the people who happen to be in town.
Art: View from a balcony, Gustave Caillebotte, 1880
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