I can't think of any tool that can do this as a simple one-off command; you could, however, convert your images individually to videos (of varying length) and then concatenate those videos together - I would use ffmpeg for this.
ffmpeg -f image2 -loop 1 -i input1.png -t 6 -crf 22 -preset veryfast 01.ts
ffmpeg -f image2 -loop 1 -i input2.png -t 9 -crf 22 -preset veryfast 02.ts
ffmpeg -f image2 -loop 1 -i input3.png -t 4 -crf 22 -preset veryfast 03.ts
-t sets the output duration in seconds. You can also use fractions of seconds, so
-t 2.2 will give you a duration of 2.2 seconds. If you wish to specify length in terms of video frames, you can use
frames:v; by default, this command will give you 25frames per second. Then, concatenate the MPEG TS files together into a single MP4:
ffmpeg -i "concat:01.ts|02.ts|03.ts" -c copy output.mp4
Of course, it would be possible to script this (I know how I'd do it in bash, but I don't have any knowledge of scripting on Windows).