Add script to generate the all-desktop.png image

Fix #27.

generate-image.sh

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+# This script automatically generates the `all-desktop.png` image, but in
+# order for it to work, you will need to have `ImageMagick Command-Line
+# Tools` installed: http://www.imagemagick.org/script/command-line-tools.php.
+
+generate_img() {
+
+    local outputImgName=""
+
+    if [ $(is_installed "convert") -eq 1 ]; then
+
+        outputImgName="$1"
+        shift
+
+        # `convert` command options:
+        # http://www.imagemagick.org/script/convert.php#options
+
+        convert $@ \
+                -background transparent \
+                -gravity center \
+                -resize 512x512 \
+                -extent 562x562 \
+                +append \
+                $outputImgName \
+        && print_success_msg "Done! 🍻 "
+
+    else
+        print_error_msg "Please install ImageMagick Command-Line Tools!"
+    fi
+
+}
+
+is_installed() {
+
+    #   0 = not installed
+    #   1 = installed
+
+    printf "$( [[ -x "$(command -v "$1")" ]] && printf "1" || printf "0" )"
+}
+
+print_error_msg() {
+    print_msg "$1"
+}
+
+print_msg() {
+    printf "\n  %s\n\n" "$1"
+}
+
+print_success_msg() {
+    print_msg "$1"
+}
+
+# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+generate_img "all-desktop.png" \
+             "chrome.png" "firefox.png" "ie-256.png" "opera.png" "safari.png"