The images are base64 encoded and not stored directly on the filesystem.
If you are looking to change this for OAuth manager and not the authorization server you will need to
1) Navigate to the policy for "oauth manager website brand logo"
2) Copy the variable assertion "brandLogo"
3) Paste this variable into "#oauth manager website brand logo"
4) Edit the variable and look for the src attribute of the image tag:
i.e:
src="data:image/png;base64,iVBORw0KGgo...."
Replace the value after
src="data:image/png;base64,
With the base64 encoded image. Be sure to keep the ending quote , tag and tag closures " /></span>
Regards,
Joe