You can load images and textures into the 3D View as stencils and project part or all of the image as paint onto your sculpted model using the Projection tool.
The stencil image is loaded into the center of the 3D View. You can scale, rotate, and tile the stencil image in its current location (in addition, you can dolly, track, or tumble the camera) to position the stencil and model in relation to each other prior to using the Projection tool.
The stencil disappears from view as you stroke, but the effect of the stencil appears on the model. To show the stencil image when you stroke, turn off the Hide When Sculpt property in the Advanced section of the Projection properties.