if this is of any help,..:
On the PSP some last files in a rco won't work,. solution>> add a dummy image after this image,.
Also some images only work when added non-compressed,. so non-inflated
-The visualization images in visualizer_plugin.rco are also non-replace-able but this could be because of checksome's in prx files,.. so mebbe the files you are trying to replace are also checked in PS3prx files,.. this would be a bummer,. because you can try and add all kinds of gim's and wouldn't work anyway,.. i think this check's are resolution based,..

Also i read there are plane png's wich you can replace in PS3 flash0,.. eg for mini's opening field,. so not all images are in rco files>?>?

i had hope you will figure this out ;) Xd,...c