After you are leaving the train you will arrive the park. Watch the cutscene and turn around, there is a big statue, look at the statue to get the option for the optional graffiti #8.Here is my complete video guide for all optional graffiti in Episode 1.
All optional graffiti are immediately saved. You can replay Episodes (Collector mode) to find your missing graffiti.
There are always 2 graffiti which you can spray, it doesn't matter which one you spray.00:08 - Optional Graffiti #1 (Recreational Vandalism)
00:56 - Optional Graffiti #2 (I See U Saw)
01:34 - Optional Graffiti #3 (The Last Unicorn)
02:22 - Optional Graffiti #4 (Home Unimprovement)
03:44 - Optional Graffiti #5 (Rock Idol)
04:36 - Optional Graffiti #6 (Dramatis Personae)
05:23 - Optional Graffiti #7 (Mulligan Stew)
06:36 - Optional Graffiti #8 (Pioneer Spirit)07:20 - Optional Graffiti #9 (Face Your Anger)
08:10 - Optional Graffiti #10 (Lucid Writing)