Upload cover images to TikTok

Uploads one or more images to the advertiser's creative library from public URLs and returns their image ids, which tiktok_add_ad and tiktok_create_video_campaign take as the ad's cover. Use when: the user wants a specific frame as the ad's cover rather than the one TikTok picks. Validate the URLs with tiktok_validate_assets first — it is free and catches the failures this cannot recover from. Do not use it for videos: video upload happens inside tiktok_create_video_campaign, which needs to attach the result to an ad in the same operation. Do not re-upload an image that is already in the library; check tiktok_list_ad_videos' sibling library in Ads Manager, or reuse the image id from an existing ad via tiktok_list_ads. Uploading changes nothing that spends money — it adds files to a library — but it is a write, so it goes through the proposal engine like every other write and is never repeated on a retry. Returns the image ids, dimensions and sizes TikTok recorded.