Here is a very quick attempt to wrap an gio outputstream around a pixbuf loader. This allows you to splice an input stream onto it, and pull a pixbuf out of the loader:

#include "gio.h"
#include <gtk/gtk.h>
#include <gdk-pixbuf/gdk-pixbuf.h>

typedef struct _GdkPixbufOutputStream GdkPixbufOutputStream;
typedef struct _GOutputStreamClass GdkPixbufOutputStreamClass;

struct _GdkPixbufOutputStream
{
  GOutputStream parent;

  GdkPixbufLoader *loader;
};

G_DEFINE_TYPE (GdkPixbufOutputStream, gdk_pixbuf_output_stream, G_TYPE_OUTPUT_STREAM)

static gssize
gdk_pixbuf_output_stream_write (GOutputStream *stream,
                                const void    *buffer,
                                gsize          count,
                               GCancellable  *cancellable,
                                 GError        **error)
{
  GdkPixbufOutputStream *pbos = (GdkPixbufOutputStream *)stream;

  if (gdk_pixbuf_loader_write (pbos->loader, buffer, count, error))
    return count;
  else
    return -1;
}

static gboolean
gdk_pixbuf_output_stream_flush (GOutputStream *stream,
                                GCancellable  *cancellable,
                                GError       **error)
{
  return TRUE;
}

static gboolean
gdk_pixbuf_output_stream_close (GOutputStream *stream,
                                GCancellable  *cancellable,
                                GError       **error)
{
  GdkPixbufOutputStream *pbos = (GdkPixbufOutputStream *)stream;

  return gdk_pixbuf_loader_close (pbos->loader, error);
}

static void
gdk_pixbuf_output_stream_finalize (GObject *object)
{
  GdkPixbufOutputStream *stream = (GdkPixbufOutputStream *)object;

  g_object_unref (stream->loader);

  G_OBJECT_CLASS (gdk_pixbuf_output_stream_parent_class)->finalize (object);

}



static void
gdk_pixbuf_output_stream_class_init (GdkPixbufOutputStreamClass *class)
{
  GObjectClass *object_class = (GObjectClass *)class;
  GOutputStreamClass *osclass = class;

  object_class->finalize = gdk_pixbuf_output_stream_finalize;
  osclass->write = gdk_pixbuf_output_stream_write;
  osclass->flush = gdk_pixbuf_output_stream_flush;
  osclass->close = gdk_pixbuf_output_stream_close;
}

static void
gdk_pixbuf_output_stream_init (GdkPixbufOutputStream *stream)
{
  stream->loader = gdk_pixbuf_loader_new ();
}


static GdkPixbufOutputStream *
gdk_pixbuf_output_stream_new (void)
{
   return g_object_new (gdk_pixbuf_output_stream_get_type (), NULL);
}


int main (int argc, char *argv[])
{
  GtkWidget *window, *image;
  GFile *file;
  GFileInputStream *in;
  GdkPixbufOutputStream *out;
  GdkPixbuf *pixbuf;

  gtk_init (&argc, &argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  image = gtk_image_new ();

  file = g_file_new_for_commandline_arg (argv[1]);
  in = g_file_read (file, NULL, NULL);
  out = gdk_pixbuf_output_stream_new ();
  g_output_stream_splice (G_OUTPUT_STREAM (out), G_INPUT_STREAM (in),
                          G_OUTPUT_STREAM_SPLICE_FLAGS_CLOSE_SOURCE|
                          G_OUTPUT_STREAM_SPLICE_FLAGS_CLOSE_TARGET,
                          NULL, NULL);
  pixbuf = gdk_pixbuf_loader_get_pixbuf (out->loader);
  g_object_ref (pixbuf);
  gtk_image_set_from_pixbuf (GTK_IMAGE (image), pixbuf);

  gtk_container_add (GTK_CONTAINER (window), image);
  gtk_widget_show_all (window);

  gtk_main ();

  return 0;
}