[size=14px]Originally submitted by Gaussian[/size]

The following technique is just one of the many techniques to convert to drawings/paintings and it will not produce the same results with every photograph.[list][*]open a file[*]create a duplicate of the layer containing the photograph[*]select this new layer[*]apply filter / stylize / find edges[*]change blending mode to "color dodge" & desaturate photo[*]change opacity to 65%[/list:u]Experiment with opacity or add a "brightness/contrast" layer style to one or both layers (use clipping group) and fiddle with the settings.
You can also use the menus "filter / artistic" or "filter / brush strokes" to add some brush stroke effects to the layer containing the "find edges" effect. It's all about experimenting and personal preference.
Note: almost the same technique can be used to bring out various details in a photograph.

Be aware(!): some shots need different blending modes, brightness/contrast and/or opacity settings.