I want to make reveal image (.png format) effect in AE; but I have encountered a problem.
I did it the same way I do when revealing letters for handwritting effect, first I drew stroke over image lines and then I just clicked on reveal. What happened was that the image didn't reveal but the stroke I drew over the image lines.
I'm not sure what your issue is, but I would approach this a little differently. You can run into size limitations for the stroke effect if you are using it to reveal a large image. The stroke may not be wide enough to cover your image.
Use the pen tool to draw a separate shape layer stroke that will draw on over your image the way you want the image revealed. You can adjust the size of your stroke to what you like. Then add the Trim Paths shape layer effect to your stroke and keyframe it to draw on over your image. Finally, place this shape layer directly above your image and set your image to Alpha Matte your shape layer.