Use a download manager such as
http://www.freedownloadmanager.org/ that will ensure the download comes down without issues, then atleast you will know if there is actually any corruption or if it is just the connection that has dropped at some point resulting in a corrupt download
No idea why Poweriso would be used at all, but I recommend you use
www.7-zip.org to extract with instead (not to avoid the above error but because it is more compatible and it does not try to extract the contents of an iso)
Testing the download myself though just incase, I am using the download manager I linked and will be using 7-zip once it downloads
EDIT: Testing Results
Main rar file verified as good and extracted just fine (used 7-zip)
The main rar contains a split archive (pspking-geb.r00 to pspking-geb.r66 + pspking-geb.rar) and a sfv file to verify the files
quicksfv verified all the files just fine
The split archive verified as good and extracted just fine (used 7-zip) [You only point to the first file for testing and extracting split files, in this case the first file is pspking-geb.rar, but sometimes that may be a *.001 file or something like that]
The final file extracted from the split archive is pspking-geb.ISO
To sum it up there is nothing wrong with the uploaded file but you may have badly downloaded this twice (I say may have because I can't figure out why you would use Poweriso with a psp disc image let alone extract with it)
I will test it on my PSP as well if you like when I free up a USB port, at the moment I am downloading direct to USB and not sure what drive relates to what USB cable as they are all the same model drives so not sure what one is free to take out at the moment (I only have 3 USB ports and I am only using 2 of my USB HDD's at the moment but have 3 plugged in, there is only one of them that I know is downloading to, the other 2, 1 is being downloaded to the other is not but not sure what port has what in out of those 2)
EDIT: ah I see they added extraction to PowerISO... I still wouldn't recommend that software, I would go as far to say stay away from it especially for console disc images and never use it to convert disc image types, most commonly messed up PSX disc images are done through PowerISO, UltraISO and other clones (It's all the same program), if you want a simple burning program then use ImgBurn that is free software and works with everything from simple PSX burns to Dreamcast burns (for converting use ISObuster [free use version is enough]), keep extracting to dedicated extraction programs such as 7-zip or WinRAR, there are a few but those 2 are the best of them and cover most commonly used formats (there are other less well known formats as well but those are hardly used or they would be well known

)