Find a photo on the internet you wish to use as your avatar by using Google search such as follows:
Save your selected image by long holding it with your finger then choosing the option "save image".
This image usually goes to an automatic folder in your phone such as "downloads" (or "screenshots" if you choose to screenshot it which uses less memory).
The image is found in your phone's gallery, often located in this app:
Then you go to your Tattle account, click on your current avatar and the "edit" icon:
These are the options that will then pop up, click "choose file":
Once you have done this, choose "gallery", then the file your desired photo is in such as "downloads" and then the photo itself.
That will change the photo in the "Avatar" box above, then click "okay" to confirm the change. (If you don't click okay, it won't change your Avatar).