Added screenshots for flatpak

lib/xdg/io.github.kotatogram.metainfo.xml

@@ -24,7 +24,16 @@
     </branding>
     <screenshots>
       <screenshot type="default">
-        <image>https://raw.githubusercontent.com/kotatogram/kotatogram-desktop/dev/docs/assets/ktg_preview.png</image>
+        <caption>Main window with default settings.</caption>
+        <image>https://raw.githubusercontent.com/kotatogram/kotatogram-desktop/dev/docs/assets/flatpak_screenshots/main_window.png</image>
+      </screenshot>
+      <screenshot>
+        <caption>Menu with Kotatogram-specific settings.</caption>
+        <image>https://raw.githubusercontent.com/kotatogram/kotatogram-desktop/dev/docs/assets/flatpak_screenshots/kotatogram_settings.png</image>
+      </screenshot>
+      <screenshot>
+        <caption>Main window with compact folders and adaptive chat bubbles enabled. Also features profile pictures rounding (currently set in JSON file).</caption>
+        <image>https://raw.githubusercontent.com/kotatogram/kotatogram-desktop/dev/docs/assets/flatpak_screenshots/customized.png</image>
       </screenshot>
     </screenshots>
     <keywords>