Wally is a freeware texture editing program. Although it has been developed for Quake2 originally, it is also very useful for Unreal. Due to its origins, it doesn't support Unreal's native file format, but only Quake's, although this isn't a problem for the Unreal-Engine designer since Wally lets you save all your work in various other standard image file formats (like .bmp, .pcx and much more), too.
According to Ty Matthews support for Unreal's native file format is currently not very high on their priority list.
Jesco: And, if I might add a comment, it wouldn't make much sense: Unreal's packages usually contain much more than only textures and the format is highly interdependent to a) objects inside themselves and b) other packages. Maintaining this interdependency would require one to write a complete implementation of the file format, which is not also virtually impossible due to its complexity but would be also very very error prone.