[Sagekilla]

that's because you're adding it back in.

If we turn the scrolling credits into a function:

function CreditScroll(clip src, string file, int "numSecs", int "vidWidth", int "vidHeight")
{
fps = src.FrameRate()
numSecs = default(numSecs, 10)
numFrames = int(numSecs * fps)

vidWidth = default(vidWidth, src.Width())
vidHeight = default(vidHeight, src.Height())

credits = ImageReader(file, end=numFrames-1, fps=fps)

return Animate(0, numFrames, "LanczosResize", credits, vidWidth, vidHeight, 0.0, 0.0, vidWidth, vidHeight, credits, vidWidth, vidHeight, 0.0, 4424.0, vidWidth, vidHeight)
}

Now let's think of this in terms of your goal. You want to give the credits after the movie:

image = "K:\AppData\Avisynth\credits2.png"

DirectShowSource("k:\AppData\VirtualDub\Class99_short.avi", fps=25.00)
Normalize(0.95)
Levels(0, 1.2, 255, 0, 255, coring=true)
Tweak(hue=0, sat=1.3, bright=0.0, cont=1.0, coring=true, sse=false)
DelayAudio(0.18)

video = last

return CreditsScroll(Video, image)

Anything seem wrong here? You're creating a clip of the scrolling credits. But that's it. You're not actually -adding- it to the original movie. To do that, you would do:

video = last
credits = ScrollCredits(video, image)
# Splicing of video and audio
return video + credits