[mariush]

Yes, both width and height must be multiples of 2, but ideally they should be multiple of 8.

as for preparing pictures... see Irfanview ... very good free image viewer. But also allows you to crop pictures or add borders.

Image menu > Resize/Resample
Image menu > Change Canvas Size or Image menu > Add border/frame

There's also a very comprehensive batch conversion tool in Irfanview (File > Batch conversion/rename) , which would allow you to resize all pictures you have to have the width or height to a particular value and keep the proportions of each picture, and also set the canvas size to a particular size, all in one go. Hit the menu entry and then check the box "Use advanced options" and then hit the Advanced button, you'll see everything you'd need there.

On my computer (fx8320 , the video was encoded at around 70fps, so about 3x real time).

As a sidenote, once you have more than a few pictures, maybe it's time to look into video editing tools

ps. add --preset ultrafast to the x264 command line and it will run like crazy ... x264.exe --tune stillimage --preset ultrafast --output "c:\path\to\output_file.mkv" "c:\path\to\avisynth_script.avs"

It sacrifices thoroughness in trying to find the best way to compress each frame in the video in favor of speed, but in your case since we're dealing with the same picture over and over, the quality will still be quite good. On my pc, the speed goes up to around 205fps on average.