A channel photo identifies the source of a message. It is different from a photograph attached to a channel post, which can carry detail and context. For the identity image, choose a compact logo, a clear portrait, or a distinctive symbol. A poster with a long channel name and several contact lines becomes difficult to understand in a small chat list.
This page uses our 1024×1024 square export. The dimensions are a preparation choice rather than an exact Telegram requirement. The useful test is whether the mark remains identifiable when it appears beside the channel name. Leave room around the outer edge so a rounded presentation does not remove part of the symbol.
If the channel covers a broad subject, choose an image that remains useful when individual posts change. A temporary announcement may work better as a post attachment than as the channel identity. Keeping the avatar stable also makes it easier for subscribers to recognise the channel without needing to read its full description every time.