I've added two strips of images after the video strip and converted their blend mode to Alpha Over. But there is still no preview image in VSE when pressing "Play". What should I do? Filetype of images is PNG, version of Blender is 2.73, graphic card is gtx860, after rendering the part of video with images I see black screen.
This was not a Blender issue.
My PNG files were created by pressing Win+PrtSc and they were not processed correctly by Blender for unknown reasons. Recreating the PNG file in another application solved my strange problem.