How to place the images inside the fire

Hi! You can do this with a Track Matte: put the fire footage directly above the letter image in the Timeline, then on the image layer, in the Modes column, use the Layer List dropdown to pick the fire layer as the Track Matte and set it to Luma Matte. That way the image only shows through the bright fire areas. If the fire footage has a black background, Luma Matte works well, since the bright fire stays opaque and the black areas become transparent. If you do not see the Modes column, right-click the timeline header area, then choose Columns, then Modes. Details: https://helpx.adobe.com/after-effects/desktop/work-with-transparency-and-compositing/work-with-track-mattes-and-traveling-mattes/track-mattes-and-traveling-mattes.html