Photos, GIFs, voice notes and video
What you can add depends on the campaign — the composer shows your options.
In the signature composer you can add blocks: text (with fonts, sizes and colors), photos, GIFs (search built in), voice notes, and on some campaigns video. Record directly in the browser — no uploads needed, though those work too.
If a button is missing, that campaign (or the organization's plan) doesn't allow that type. Length and size limits come from the campaign; the composer enforces them as you go.