The .scr file format
A .scr (screen) file is a memory dump of the Spectrum's display file, or screen memory. This file format is supported by many emulators.
The display file starts at address 16384 and is 6912 bytes long. The first 6144 bytes represent the actual screen bitmap (which pixels are set and which are not), and the remaining 768 bytes represent the attributes, or colour information, which can be imagined as a transparent overlay.
The layout of the display file is somewhat unusual. It is split into vertical thirds, each comprising eight rows of characters, and each character is stored in eight (non-sequential!) bytes. The attribute data is stored in a more straightforward way, "reading" the character cells from the top left to the bottom right.
This simple program illustrates the layout of the screen display by filling it with consecutive bytes:
10 FOR f=0 TO 6911 20 POKE f+16384, 255 30 NEXT f