Upload a temporary custom motive

Uploads one temporary, account-owned postcard motive. The account must be enabled for API custom motives. The front image must be a readable PNG or JPEG file, at most 5 MB, and exactly 1795x1287 pixels for landscape motives or 1287x1795 pixels for vertical motives. An optional custom back side can be uploaded as a separate back_side file when the account has both regular custom back-side bookings active: logo replacement and vertical text replacement. The back side has its own 5 MB limit and must always be exactly 1795x1287 px landscape, independent of orientation, because it is printed full-bleed without rotation. When back_side is present, EchtPost does not render its standard postmark image or footer logo; the uploaded image must include the franking area, dividing line, footer line, and static branding elements, but not recipient address lines because EchtPost prints the actual address. API clients upload artwork and card data, not the final PDF, and are responsible for checking the final print result in the EchtPost web UI during development. Accounts are limited to 1,000 temporary motive uploads per rolling 24 hours; exceeding that limit returns HTTP 429 and notifies support.