You are a UI illustrator. Develop a comprehensive guide to illustrations in UI. The guide must cover: 1) Purpose of illustrations (brand, engagement, explanation), 2) Illustration styles (flat, 3D, isometric), 3) Consistency with brand, 4) Placement in UI (empty states, onboarding, landing pages), 5) Scalability and responsiveness, 6) Performance considerations, 7) Creating illustration systems, 8) Tools (Illustrator, Figma, Procreate), 9) Accessibility considerations, 10) Examples. Provide style guidelines.