I'm not a fan of C style libraries and would like to know if there is something like GEGL ( http://www.gegl.org/ ), with a more C++ friendly design.

It need not be as mature and feature full as the mentioned one, just enough to do some image processing, without too much off-hand theoretical knowledge.

Not sure how much work you need to do to images here, but you may wish to look at kdcraw.

I really depends on what your trying to do and what features you need, but I would start with ...

Magick++ http://www.graphicsmagick.org/Magick++/
I supports 88+ Image formats, has 50+ effects, ..... Its Gimp in a easy to use C++ API
Example Code: http://trac.imagemagick.org/browser/ImageMagick/trunk/Magick%2B%2B/demo/demo.cpp
Magick++ Effects List

adaptiveThreshold
addNoise
addNoiseChannel
affineTransform
annotate
blur
blurChannel
border
cdl
channel
channelDepth
charcoal
chop
colorize
colorMatrix
comment
compare
composite
contrast
convolve
crop
cycleColormap
despeckle
display
draw
edge
emboss
enhance
equalize
erase
flip
floodFillColor
floodFillOpacity
floodFillTexture
flop
frame
gamma
gaussianBlur
gaussianBlurChannel
implode
haldClut
label
level
levelChannel
magnify
map
matteFloodfill
medianFilter
minify
modifyImage
modulate
motionBlur
negate
normalize
oilPaint
opacity
opaque
quantize
quantumOperator
process
raise
randomThreshold
randomThresholdChannel
reduceNoise
roll
rotate
sample
scale
segment
shade
sharpen
sharpenChannel
shave
shear
solarize
spread
stegano
stereo
strip
swirl
texture
threshold
transform
transparent
trim
type
unsharpmask
unsharpmaskChannel
wave
zoom

If you need advance color management for HDR Images Note: Every Pro Image Editing Related Software in the Industry uses these two.
OpenColorIO & OpenImageIO http://opencolorio.org/ and https://sites.google.com/site/openimageio/

For handling RAW Images specifically you can use KDcraw http://api.kde.org/4.x-api/kdegraphics-apidocs/libs/libkdcraw/libkdcraw/html/index.html

For "Image Plugins" there is Kipi http://extragear.kde.org/apps/kipi/ and http://api.kde.org/4.x-api/kdegraphics-apidocs/libs/libkipi/libkipi/html/index.html Kipi does a bunch of Photo related stuff

NEW FlickrExport: export images to a remote Flickr web service.
NEW SimpleviewerExport: export your images in a nice flash movie.
NEW HTMLGallery: export your images to HTML
RawConverter: A raw image converter for digital cameras.
SlideShow: Slideshow with effects ripped out from kslideshow and 3D effects using OpenGL.
MpegEncoder: Create an MPEG slideshow from your images.
PrintWizard: A wizard to print images in various format.
JpegLossLess: Batch process your JPEG images without losing meta information and compression.
CdArchiving: Archive your albums on CD or DVD using K3b.
ScanImages: Scanner management using Kooka.
ScreenshotImages: Snap screen based on KSnapshot and adapted to Kipi.
Calendar: A plugin to create calendars.
SendImages: A plugin to send images by email, allowing resizing and recompressing before sending.
RenameImages: Batch image renamer.
ConvertImages: Batch image converter.
BorderImages: Add border to your images in batch.
FilterImages: Batch image enhancer using digital filters.
ColorImages: Batch image color enhancer.
EffectImages: Batch image transformation effects.
ResizeImages: Batch image resizer.
RecompressImages: Batch image recompressor.
FindDuplicateImages: Find duplicate images in albums.
WallPaper: Set your image as wallpaper.
TimeAdjust: Adjust image file time and date.
GalleryExport: Interface for export images collections to remote Gallery web server.
KameraKlient: Digikam 0.6.2 Gphoto2 interface ported as a Kipi plugin. Note: this plugin is disabled in digiKam release >= 0.7.x because this part have been re-written from scratch using KIO Slaves.

You can also use OpenGL Shaders and or Projection on a Texture using Qt if you need something special.

And there is a GEGL C++ Qt Api also but it only just started. http://www.ohloh.net/p/gegl-qt