Incorrect (depending on what you call "picture files" (I'm assuming you're talking about Image Management images). These are all stored in the config.idb database and these are Gateway-scoped, not project-scoped.
All configuration-related files are stored under here (for Windows): C:\Program Files\Inductive Automation\Ignition\data
Perspective project files are stored under: .\projects\<ProjectName>\com.inductiveautomation.perspective\...
Perspective theme files, icons, and fonts are stored under: .\modules\com.inductiveautomation.perspective\...
The config.idb is stored under: .\db\...
You can get the data directory if you run this from the gateway scope (which includes from a Perspective session):
from com.inductiveautomation.ignition.gateway import IgnitionGateway
context = IgnitionGateway.get()
projectParentFolder = str(context.systemManager.dataDir.absoluteFile).replace('\\','/')
Or, if you've got the Ignition Extensions module installed:
system.util.getContext().systemManager.dataDir.absoluteFile.replace('\\', '/')