Set up frames for your thumbnails

Use Quickshape to draw straight lines from corner to corner, creating an X as a guide. Next, use Options> Canvas>drawing Guide to create your frames. Enable the drawing guide and tap Edit Drawing Guide below that. The 2D Grid settings are perfect. You can enable Assisted Drawing by clicking a layer and selecting Drawing Assist. Draw vertical and horizontal lines for your frames using the diagonal lines as guides to keep your proportions true.