Here is a thorough tutorial on making speech bubbles in photoshop. You can use gimp if you know how to use it&amp;#8217;s path system (you&amp;#8217;ll have to do that research on your own).
Here&amp;#8217;s a link to the original size, and here&amp;#8217;s tumblr&amp;#8217;s scaled down &amp;#8220;hi-res&amp;#8221; version if that first link dies for whatever reason.