Added download_daemon which needs to be running in background. Added downloader.php - the part of program that interacts with Unsplash API. Added lock which locks the screen. Added configs.

box_config

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+BOX_TOP_LEFT_X=10
+BOX_TOP_LEFT_Y=610
+BOX_BOTTOM_RIGHT_X=410
+BOX_BOTTOM_RIGHT_Y=720
+ROUNDENESS=50
+NAME_X=600
+NAME_Y=10

config

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+# TIMECOLOR="00FFFFFF"
+# TIMEFONT="Neuropolitical"
+# TIMESIZE=60
+# TIMESTYLE="%H:%M:%S"
+# DATECOLOR="00FFFFFF"
+# DATEFONT="Space Age"
+# DATESIZE=25
+# DATESTYLE="%A, %d %b"
+## Ring Colors
+# VERIFYING_INSIDE="FFFFFF44"
+# WRONG_INSIDE="FF000044"
+# INSIDE="00000000"
+# LINE="00000000"
+# KEYPRESS="FFFFFFFF"
+# BACKSPACEPRESS="CCCCCCFF"
+# RINGCOLOR="F7045DFF"
+# RINGVERIFYINGCOLOR="F7045DFF"
+# RINGWRONGCOLOR="F7045DFF"
+# RINGTEXTSIZE=14
+# RINGRADIUS=45

download_daemon

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+#!/usr/bin/env bash
+# Get the config
+source box_config
+
+# Get the resolution
+resolution=`xdpyinfo | grep dimensions | sed 's/dimensions://;;s/ *//g;s/pixels.*//'`
+
+# Run the main loop
+while : ; do
+	# Check whether the previous image has been used or not.
+	# If no, wait 300 secs and check again.
+	[[ `< $HOME/.config/lock_screen.sta` -eq 0 ]] && sleep 300 && continue
+	# Download the new image
+	$HOME/i3lock-delta/downloader.php
+	if [[ $? -eq 0 ]]; then
+		name_photographer="`< /tmp/name_photographer`"
+
+		# Resize, crop, draw a box for clock, write photographer's name.
+		convert $HOME/.rand_bg -resize "$resolution^" -crop "$resolution" \
+				-draw "fill black fill-opacity 0.4  roundRectangle
+				$BOX_TOP_LEFT_X,$BOX_TOP_LEFT_Y $BOX_BOTTOM_RIGHT_X,$BOX_BOTTOM_RIGHT_Y
+				$ROUNDENESS,$ROUNDENESS text $NAME_X,$NAME_Y 'Photograph by: $name_photographer on Unsplash'" \
+				/tmp/temp_lock.png
+
+		# Move to needed place and remove the rest.
+		if [[ -f /tmp/temp_lock-0.png ]] ; then
+			mv /tmp/temp_lock-0.png $HOME/.rand_bg.png
+			rm /tmp/temp_lock-1.png
+		fi
+
+		# Remove the original
+		rm $HOME/.rand_bg
+		# Set the image as unused.
+		echo 0 > $HOME/.config/lock_screen.sta
+	else
+		# If download fails, wait 60 secs and restart.
+		sleep 60
+	fi
+done

downloader.php

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+#!/usr/bin/env php
+<?php
+require __DIR__.'/vendor/autoload.php';
+
+# Initialize API
+Crew\Unsplash\HttpClient::init([
+	'applicationId' => file_get_contents(getenv('HOME').'/i3lock-delta/api_key'),
+	'utmSource' => 'i3lock-Delta'
+]);
+
+# Get custom search queries
+$queries_file_name = getenv('HOME') . '/.config/lock_preferences';
+$queries = file($queries_file_name);
+
+# Create the random photo limitations
+$filters = [
+	'featured' => false,
+	'query' => $queries[rand(0,count($queries) - 1)],
+	'orientation' => 'landscape'
+];
+
+# Get the random photo
+$img = Crew\Unsplash\Photo::random($filters);
+
+# Download the photo
+$url = $img->download();
+$output = file_get_contents($url);
+
+# Write image to a file
+$image_filename =getenv('HOME') . '/.rand_bg';
+$image_file = fopen($image_filename, 'w');
+fwrite($image_file, $output);
+fclose($image_file);
+
+# Write author's name to a file
+$name_file = fopen('/tmp/name_photographer', 'w');
+fwrite($name_file, $img->{'user'}['name']);
+fclose($name_file);

lock

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+#!/bin/bash
+TIMECOLOR="00FFFFFF"
+TIMEFONT="Neuropolitical"
+TIMESIZE=60
+TIMESTYLE="%H:%M:%S"
+DATECOLOR="00FFFFFF"
+DATEFONT="Space Age"
+DATESIZE=25
+DATESTYLE="%A, %d %b"
+# Ring Colors
+VERIFYING_INSIDE="FFFFFF44"
+WRONG_INSIDE="FF000044"
+INSIDE="00000000"
+LINE="00000000"
+KEYPRESS="FFFFFFFF"
+BACKSPACEPRESS="CCCCCCFF"
+RINGCOLOR="F7045DFF"
+RINGVERIFYINGCOLOR="F7045DFF"
+RINGWRONGCOLOR="F7045DFF"
+RINGTEXTSIZE=14
+RINGRADIUS=45
+
+# Get custom config
+source $HOME/i3lock-delta/config
+
+# Pause dunst
+pkill -u ahmad_saalim -USR1 dunst
+# Set the image as used.
+echo 1 > $HOME/.config/lock_screen.sta
+xset dpms force off
+
+# Call the lock
+i3lock -i $HOME/.rand_bg.png -knf \
+    --timecolor="$TIMECOLOR" --layout-align 1 --time-align 1 --timefont="$TIMEFONT" \
+    --datecolor="$DATECOLOR" --timepos "30:h-ch" --datesize="$DATESIZE" \
+    --timesize="$TIMESIZE" --datestr="$DATESTYLE" -e --datefont="$DATEFONT" \
+    --date-align 1 --insidevercolor="$VERIFYING_INSIDE" --insidewrongcolor="$WRONG_INSIDE" \
+    --insidecolor="$INSIDE" --linecolor="$LINE" --keyhlcolor="$KEYPRESS" --bshlcolor="$BACKSPACEPRESS" \
+    --ringcolor="$RINGCOLOR" --ringvercolor="$RINGVERIFYINGCOLOR" --ringwrongcolor="$RINGWRONGCOLOR" \
+    --separatorcolor="$KEYPRESS" --indpos="w-r-30:h-r-60" --radius="$RINGRADIUS" --modsize=1 \
+    --textsize="$RINGTEXTSIZE" --indicator --timestr="$TIMESTYLE"
+
+# Resume dunst
+pkill -u ahmad_saalim -USR2 dunst