In StashApp you can customise the image of your account, you can choose any of the NFT's in your wallet.
Go to your account tab, you can bring up the collection overview by tapping the avatar at the top of the screen, see the images below:
Then tap the account avatar image as in the image below:
From there select one of the NFTs in your list as in the image below:
When prompted to set the NFT as wallet NFT, tap on YES.