The PSP's flash contains numerous RCO files (localized resources)
in the flash0:/vsh/resource folder.  These files contain various
things such as icons, sounds, text and so on, which the PSP uses to
display the XMB interface.
RCO Editor is what its name implies.  Currently almost all of the
RCO can be edited in RCO Editor.  Some example uses:

• replace icons
  
• replace sounds - support for both stereo and mono sounds
  
• replace text (longer text is supported)
  
• modify some page and anim data

Random update on 18th October 2009
More bugs fixed - crash found thanks to byte.me, and some 4 bit conversion issues.  
You can get the updated EXE here - just replace the EXE in the package below with the one in the link in this sentence.

Random update on 11th June 2009
A number of bugs fixed in 1.15c.  I'm lazy so won't update the version number (actually, I'm getting close to finishing my rcomage program which will replace RCO Editor to some degree).  You can get the updated EXE here - just replace the EXE in the package below with the one in the link in this sentence.
This update just fixes a few issues with image handling (more specifically GIM<->BMP conversion).


Last updated, 28th Oct, 2007, version 1.15c
Changes in 1.15c

• Fixed another bug in GIM conversion routine

Changes in 1.15b

• Fixed a bug in the decompression and GIM conversion routine

Changes in 1.15:

• Rewrote GIM <-> BMP conversion routine
  
  • Dimension restrictions removed, as well as the "visible dimension" nonsense (RCO Editor automatically fixes that)
    
  • osk_utility.rco icons no longer cause crashes
    
  • Support for creating 4 bit and 32 bit GIMs (previously, only 8 bit supported) - effectively removing the 256 colour restriction
    
  • You can now use ICO, PNG and GIF files as inputs, in addition to 4 bit BMPs
• Other minor changes and bug fixes
  
• Added option to disable compressing resources - this is for some 3.7x RCOs not working properly with compression enabled

So what sort of things can this program achieve?  Well, you can just take a look around the many themes which are now possible with this.  You can find a list of some themes here.

Okay, that out of the way:
Download (142KB)

Source code available here

neat

W00t, alpha 2 is out now!  You can now view the icons, and extract BMPs of them.  Only a short time till I make a BMP -> MIG converter, so you can use your own icons :)

Yay, I got the BMP -> MIG converter done!  Now you can change your OWN icons!  Check out the custom battery icon (yeah it sucks - my graphics skills are crap).
Also take a look at the custom coldboot and gameboot branding :)

why is the thumbnail not clickable :/ i can't see chocolate

My bad, sorry.  Fix'd :)

Basically any icon can be changed.  I used the battery as an example.  You can change the busy indicator, the coldboot and gameboot brandings, even the "Pop"/"Jazz" etc icons when you press the Note button.

Ooh, just upgraded to 3.10 OE-A, and guess what?  RCO customization still seems to work!

Lol, it seems that Sony just blocked topmenu_plugin.rco customization, but nothing else :S  Interestingly, even an encrypted 3.03 topmenu doesn't work on 3.10.

Oh well, anyways, it means you can change your battery icon (and many others) on 3.10 OE-A! :)

Hehe, another release.  This will now be able to swap OMG and VAG files :)

Also, I tested the opening_plugin.rco again without padding - the sound now works :), but for some reason, the animation is screwed.

Oh, another thing, as the program now handles OMG and VAG files, you're no longer limited to the ~100KB VAG size restriction :D

I think this is good enough to start posting this on other forums XD