There are a bunch of different plugins that can do screenshots. The one I'm using right now is called prxshot.prx
You can download it here:
http://github.com/downloads/codestation/prxshot/prxshot-0.4.0.zip
To install:
1) Unzip, hook up PSP to USB
2) Copy the prxshot folder to Memory Stick:/seplugins/ -- if there is no seplugins folder, create it.
3) Inside seplugins folder, create game.txt, pops.txt, vsh.txt if any of the three don't exist. game.txt tells the PSP which plugins to use inside PSP games. pops.txt tells the PSP which plugins to use inside PS1 games. vsh.txt tells the PSP which plugins to use in the OS.
If you are putting the plugin on the PSP's internal storage (IE with a PSP go), write the following in all three files:
ef0:/seplugins/prxshot/prxshot.prx 1
If you are putting the plugin on the PSP's memory stick, write the following in all three files:
ms0:/seplugins/prxshot/prxshot.prx 1
Just that one line. Nothing before, nothing after.
4) Save the files. Reboot PSP. Remember to use the Pro Restore again.
To take a screenshot:
1) Press the music note button. Screenshots will shot up in PSP photos or in Memory Stick:/PSP/SCREENSHOT
To verify which plugins are running:
1) Make sure you're using CFW
2) Press select in the XMB
3) Select Recovery Menu
4) Select Plugins
5) Select either System Storage if you're using a PSP that has internal storage or Memory Stick if you're using a PSP that doesn't. You'll see the plugins there.