aru has released a new version of his SparkIV tool, which enables GTA IV IMG/RPF archives to be imported and exported. The major feature in this new release is basic texture importing with full MipMap generation support. Here is the full list of new features:

• Support for Texture File editing with Import (replace only) functionality.
• Support for viewing Texture mipmaps and viewing colour channels seperately
• Support for showing most of the filenames in Audio RPF (v3) files.
• Support for launching external registered editors based on extension.
• Fixed some bugs with IMG file saving.

And here's what some of our crazy modders have been doing with it so far...

Texture replacement in GTA IV (PC) using SparkIV's new import feature.

Related Link: GTAForums Topic (download link is in the first post)