Watermark on an image

[msroid]

I used this command to add a watermark on an image:

ffmpeg -i input.png -i watermark.png -filter_complex overlay=15:15 output.png

But I got this error:

[NULL @ 0x7a1403f800] Unable to find a suitable output format for '/storage/emulated/0/images/img_wat.png'

I already add a watermark on a video but I don't know how to do that in this case for an image instead of a video.

Could someone help me please?

[msroid]

Please enable image2 and segment muxers!

[Aracem]

FYI @msroid I use image2 command to create a slideshow of images in my app. Also, put a Watermark over a video as you commented.

[msroid]

@Aracem I can put watermark on a video but it doesn't work when I try to put a watermark on an image! Have you tried to put a watermark on an image?