I'm using the latest texsync'd version of ConTeXt on 32bit and 64bit
linux. Looks like either ConTeXt or pdfetex can't handle PNG's with an
alpha channel.

PNG image data, 8-bit/color RGB, non-interlaced  <-- works fine
PNG image data, 8-bit/color RGBA, non-interlaced <-- corrupt pdf
PNG image data, 8-bit gray+alpha, non-interlaced <-- following error:
---
libpng warning: Ignoring bad adaptive filter type
libpng warning: Ignoring bad adaptive filter type
libpng warning: Ignoring bad adaptive filter type
---
repeated ad nauseum

(To benefit anyone coming across this message in the archives:)
As a work around, assuming you don't need the alpha, the image can be
converted to RGB in The Gimp using Layers > Flatten Image.
Alternatively, using Image Magick, the command would be
convert image.png png24:image-new.png

-- 
Stuart Jansen <sjansen@gurulabs.com>
Guru Labs, L.C.