You need to set home domain for your issuer account and host valid https://<domain>/.well-known/stellar.toml file on your domain with image=<logo url>. And don't forget about CORS headers, otherwise browser won't be able to access your stellar.toml file
According to documentation here:
https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0001.md#currency-documentation
Here's simple example:
[[CURRENCIES]]
code="BTC"
issuer="GAUTUYY2THLF7SGITDFMXJVYH3LHDSMGEAKSBU267M2K7A3W543CKUEF"
display_decimals=7
name="Bitcoin"
desc="1 BTC token entitles you to receive 1 Bitcoin (minus outgoing transaction fee) to an address of your choice using our anchor."
conditions="Outgoing transaction fee deducted from the amount sent"
image="https://apay.io/public/logo/btc.svg"