I'm fuzzy on the relationship between Windows Imaging Component (WIC) and GDI+. I've done some work in the past that showed that, for example, WIC produces visually superior GIF encodings, but I'm surprised I don't see more people using it for image processing vs. GDI+. I know it doesn't have GDI+'s draw operations, but for encoding/decoding it seems superior. So why don't we see a migration?
The relationship (or rather the difference between) WIC and GDI+ is that WIC is an extensible imaging codec framework which allows applications implementing the framework to receive support for new image formats via provided codecs. GDI+ is a core component of Windows which supports draw operations such as lines, fonts, gradients, etc.
While GDI+ has native support for several common image formats, WIC codecs can be provided for any image format.