Pixelmator acts as an image editor that does more than an image editor. It features ways to modify the images and content in those images as well as adding elements (eg. shapes) to customize them. You can use this for reference to create thought bubbles, although you'll have to work out a little to get this done.
I suggest checking the available data on the official website. Even if it's briefly explained, you'll find enough information or get a glimpse of what Pixelmator can do.