How can I see a preview of the sequence of PNGs in the VSE? I want to create a video from a png image sequence.
I have imported all images to Blender:



But I can´t see a video preview as shown in the above image. 

How can I see a preview of the video's output? 

This are the properties for the first image file:



I have tried advancing frame to check unsuccessfully. 

I have added 1 frame to check without success. Same with Alpha over/Under.

All png have alpha channel.
Could be that you are looking at the wrong frame (frame zero instead of frame 1). Try advancing a frame to check.

Also check that the png is being interpreted by Blender correctly. Import just 1 frame and check.

Finally you could try using Alpha Over or Over drop in the strip's blend type. It is at the top of the strip properties bar on the right of the VSE.