Remove custom material

[MR H 91]

Hello

I've searched the forum for an answer, and have found a couple of posts that are similar to my request, but there wasn't a solution to my problem.

I'm fairly new to Sketchup, I have SU Make 2017. I was messing around with custom colours and textures the other day, and I stupidly made an OSB wood texture using a PNG image that I designed in Illustrator. The file is HUGE, and it slows my MacBook down even when opening the "Wood" folder in the materials list (see thumbnail image below).

When I right click and delete the texture from the "Wood" library, it deletes, as expected. However, when I quit Sketchup and re-open it, the texture is there again!

I have tried time and time again to find the folder on my MacBook in which the image is stored. There is nothing showing in SketchUp 2017>Materials (other than some .skm textures downloaded from sketchUcation) see below.

Essentially, I'm asking where am I able to find this image now stored in Sketchup, and how do I permanently delete it?

Thanks in advance.