[FFmpeg-user] creating a slideshow

[gheine at mathnmaps.com]

Hello -

Have a number of still pictures (jpg) that I would like to make into a  
slideshow.

Using information at < https://trac.ffmpeg.org/wiki/Slideshow >,  
created a file "input.txt" with the format:

first_image.jpg
duration 5
second_image.jpg
duration 5
....
last_image.jpg
duration 5
last_image.jpg

then ran the command:

ffmpeg -f concat -i input.txt -vsync vfr -pix_fmt yuv420p test.mp4

This did work.  "test.mp4" is a video with all the images, each  
displaying for about 5 seconds.  But ffmpeg gave the following  
complaint for each image:

[swscaler @ 0x9d13040] deprecated pixel format used, make sure you did  
set range correctly

Tried the command without "-pix_fmt" ; gave same result
Tried the command without either "-vsync" or "-pix_ftm"; also gave  
same result, but processing was much slower
Then I converted all the jpg images to png and ran the command again.   
Same result.

Questions: what is a "deprecated pixel format"?  What does it mean to  
"set range" ?
What negative effects, if any, might be expected from these warnings?

thanks for any information -