Try to find the gd.so file used by php for the gd functions (if it is installed with ubuntu packages it should be in /usr/lib/php5/20060613) and type:
ldd gd.so
you will see all shared libraries that the object is using:
linux-vdso.so.1 => (0x00007fff8f7ed000)
libgd.so.2 => /usr/lib/libgd.so.2 (0x00007f813cfcc000)
libt1.so.5 => /usr/lib/libt1.so.5 (0x00007f813cd6f000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x00007f813caea000)
libX11.so.6 => /usr/lib/libX11.so.6 (0x00007f813c7de000)
libXpm.so.4 => /usr/lib/libXpm.so.4 (0x00007f813c5cd000)
libpng12.so.0 => /usr/lib/libpng12.so.0 (0x00007f813c3a7000)
libz.so.1 => /usr/lib/libz.so.1 (0x00007f813c190000)
libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x00007f813bf6e000)
libc.so.6 => /lib/libc.so.6 (0x00007f813bc1a000)
libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0x00007f813b9e5000)
libm.so.6 => /lib/libm.so.6 (0x00007f813b762000)
libxcb-xlib.so.0 => /usr/lib/libxcb-xlib.so.0 (0x00007f813b560000)
libxcb.so.1 => /usr/lib/libxcb.so.1 (0x00007f813b344000)
libdl.so.2 => /lib/libdl.so.2 (0x00007f813b140000)
/lib64/ld-linux-x86-64.so.2 (0x00007f813d43a000)
libexpat.so.1 => /usr/lib/libexpat.so.1 (0x00007f813af16000)
libXau.so.6 => /usr/lib/libXau.so.6 (0x00007f813ad14000)
libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0x00007f813ab0f000)
libpthread.so.0 => /lib/libpthread.so.0 (0x00007f813a8f2000)
Check if libjpeg heads to the right version. If not, you should figure out why (hint LIBRARY_PATH)