Step 1. Download all the CSP Portraits you want to replace, save them in a folder you can find easily later on.
Step 2. Open your "sc_selcharacter.pac" in BrawlBox.
Step 3. Go to "char_bust_tex_lz77" in your "sc_selcharacter.pac" and open this too.
Step 4. Now go to the character's CSP Portrait you want to replace. (Detailed information on finding your character in the spoiler ahead.
Step 5. Right click on the CSP Portraits you want to replace and select "replace". Guide it to the map where you saved your CSP Portraits and select the correct CSP Portrait to replace with.
Step 6. After you're done replacing all of your CSP Portraits you can save your file and put it on your SD card.
If anything is unclear about this please let me know!