Hi Marc,
The TextViewDrawContext tries to use core GDI APIs as much as possible since they are very fast compared to GDI+, and they render text spacing correctly. I believe what's happening is that the TextViewDrawContext is in GDI mode when it enters here. At least by the official release of build 400, we call the AlphaBlend API to render images. That should handle semi-transparent images ok.
Can you e-mail our support address with the image you're trying to draw so we can try the same here and see the issue happening? Please reference this thread in your e-mail.
Another workaround you could try is to flip to GDI+ temporarily. Toggle the context.IsNativeRendering to false, draw the image, and then flip it back to true. I haven't tried that myself but it may help in the meantime. It would be better to sort out why it's not working in the first place.