Life is Strange: Before the Storm

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)

This achievement is earned for finding optional graffiti #8. If missed on the first run, you can choose the episode from the main menu and select "Collector Mode". Now, you can use a scene select option to browse the locations and choose the area you need.

The following collectible can be found after the train section. After gaining control of your character on top of the mountain, turn around the notice the large statue and tag the bottom of it.
[5:05] - Optional Graffiti #8