System Information
Windows 10, NVIDIA GeForce GTX 660 Ti
Blender Version
Broken: 2.75a c27589e
Worked: 2.74
Short description of error
If you try to replace an image that has been packed into the .blend file it won't replace the image.
Exact steps for others to reproduce the error
Here's a .blend file with a packed image for example:
https://dl.dropboxusercontent.com/u/108488518/Replace_Image_Bug.blend
Or just load up a new .blend file drag in an image into the image editor and pack it into the .blend file.
Then go to: Image > Replace Image (in the image editor)
http://i.imgur.com/3eGlC1h.png
Chose any image on your computer and try to replace it with the packed image and the image won't change.