docs: layout specifix collage frames & backgrounds

Change-Id: I0a4add8906d495cf649fdd3fa2f07567e8b1d047

Author: andi34

docs/faq/custom-collage.md

@@ -129,3 +129,74 @@ Notes:
 - `placeholder` uses `placeholderpath` at `placeholderposition`.
 
 Single framed images only work when `apply_frame` is `always`.
+
+## Collage Frames and Backgrounds: Name Changes
+
+This guide explains how layout-specific frames and backgrounds are applied automatically if they exist. The system supports **default templates** with specific layout prefixes.
+
+---
+
+### How Name Changes Work
+
+When you select a collage layout, the system looks for **layout-specific files**:
+
+- **Frames:** `<layout>_<frame_filename>`
+- **Backgrounds:** `<layout>_<background_filename>`
+
+If the layout-specific file exists, it replaces the default. If it does **not exist**, the system uses the default file.
+
+**Example:**
+
+- Default frame: `basic.png`
+- Layout: `1+2-1`
+- System checks for: `1+2-1_basic.png`
+  - If exists → used automatically
+  - If not → falls back to `basic.png`
+
+---
+
+### Supported Layout Prefixes / Default Templates
+
+The following prefixes can be used when naming layout-specific frame or background files (based on default available templates):
+
+- `1+2-1`
+- `1+3-1`
+- `1+3-2`
+- `2+1-1`
+- `2+2-1`
+- `2+2-2`
+- `2x3-1`
+- `2x3-2`
+- `2x4-1`
+- `2x4-2`
+- `2x4-3`
+- `2x4-4`
+- `3+1-1`
+- `collage`
+
+---
+
+### User Guidelines
+
+1. **Upload layout-specific files** using the pattern: `<layout>_<filename>`
+   Example: `2+2-1_frame.png` or `2+4-2_background.jpg`
+
+2. **Keep default files** in place for fallback.
+   Example: `frame.png`, `background.jpg`
+
+3. **Automatic selection:**
+   - When a layout is selected, the system checks if a layout-specific file exists.
+   - If it exists, it replaces the default automatically.
+   - If not, the default is used.
+
+---
+
+#### Example Workflow
+
+| Selected Layout | Default Frame | Layout-Specific Frame Used |
+|-----------------|---------------|---------------------------|
+| `2+2-1`         | `basic.png`   | `2+2-1_basic.png`         |
+| `2x4-2`         | `basic.png`   | `2x4-2_basic.png`         |
+
+- Backgrounds follow the **same naming convention**.
+  Example: `2x3-1_white.png` replaces `white.png` if it exists.