It is not provided as a feature in the app.
There is a work around to it.
Below are the steps to create a cloud shaped call out.
1) Create a empty canvas
2) Use the shape crop (cloud kind of shape)
3) Then save the picture
4) Create another empty canvas
5) Load the first file with "Add photo"
6) Draw the 3 rings that come below the cloud
7) Now save the picture
You can use the finally saved picture as call out. Just use add photo and select the file you created.
The above steps can be replicated for any number of call outs in any shape.
Hope this helps.