This is from an old notebook of mine. The girl from Ipanema, composed of 35,000 points. (Note true pointillism, perhaps someone can do that one as well.) Sorry about the messy code and the slow processing.
The idea is to randomly scatter disks in a rectangle, and colour them according to the corresponding points of the photo. More points with a higher density are used in regions of high detail or sharp transitions, fewer points elsewhere (to keep their number down).
img = Import["http://farm1.staticflickr.com/125/409696380_05f5c89a37_b_d.jpg"] 
etf = EntropyFilter[img, 12] // ImageAdjust sdf = ColorConvert[StandardDeviationFilter[img, 5], "GrayScale"] // ImageAdjust map = ImageAdd[sdf, etf] // ImageAdjust 
mapdata = ImageData[map]; data = ImageData[img]; {w, h} = ImageDimensions[img]; ch = RandomChoice[ (Flatten[mapdata] + 0.1)^1.7 -> Join @@ Table[{i, j}, {i, h}, {j, w}], 35000]; spots = {data[[#1, #2]], {#2, -#1}, 15 (1.1 - mapdata[[#1, #2]])^1.8} & @@@ ch; spots = Reverse@SortBy[spots, Last]; Graphics[{RGBColor[#1], Disk[#2, #3]} & @@@ spots, Background -> GrayLevel[0.75], PlotRange -> {{1, w}, {1, -h}} ] 
Another example (unfortunately I lost the original source photo and it's not as easy to google up as the other one above):
