Redmine

Almost the same to me. I can see some images in pdf but not all of them:

$ file *
img1.png: PNG image, 26 x 25, 8-bit/color RGB, non-interlaced
img2.png: PNG image, 24 x 24, 8-bit/color RGBA, non-interlaced
img3.png: PNG image, 24 x 24, 8-bit/color RGBA, non-interlaced
img4.png: PNG image, 833 x 945, 8-bit/color RGBA, non-interlaced
img5.png: PNG image, 47 x 47, 8-bit colormap, non-interlaced
img6.png: PNG image, 24 x 24, 8-bit/color RGBA, non-interlaced

Environment:
  Redmine version                2.3.2.stable.12078
  Ruby version                   1.9.3-p385 (2013-02-06) [x86_64-linux]
  Rails version                  3.2.13
  Environment                    production
  Database adapter               SQLite
Redmine plugins:
  redmine_bbb                    0.1.2
  redmine_people                 0.1.6

Tried pngs without alpha channel (got from here http://www.fnordware.com/superpng/samples.html) - the same result - no images in PDF export.

I tried note-8 URL Images.
The following error occurred.

pdf: Image: error: TCPDF error: Alpha channel not supported: /opt/Redmine/redmine.d/redmine-2.2.3/files/130805004924_pngtest8rgba.png
pdf: Image: error: TCPDF error: 16-bit depth not supported: /opt/Redmine/redmine.d/redmine-2.2.3/files/130805004924_pngtest16rgba.png
pdf: Image: error: TCPDF error: 16-bit depth not supported: /opt/Redmine/redmine.d/redmine-2.2.3/files/130805004924_pnggradHDrgba.png
pdf: Image: error: TCPDF error: 16-bit depth not supported: /opt/Redmine/redmine.d/redmine-2.2.3/files/130805004924_pnggrad16rgb.png
pdf: Image: error: TCPDF error: 16-bit depth not supported: /opt/Redmine/redmine.d/redmine-2.2.3/files/130805005029_pnggrad16rgba.png

Sorry, RFPDF (TCPDF) library is not supporting 16-bit depth PNG image.