LinkedIn video ad thumbnail and captions: JPG/PNG 2 MB, SRT

LinkedIn's video ad page asks for an optional JPG or PNG thumbnail of at most 2 MB, and for captions as an SRT file with text only. Sume can cut a still from your clip with POST /v1/video-frames, but its caption route burns text into the picture; the docs describe no SRT export.

Specs are from LinkedIn's Marketing Solutions Help page and Sume's video frames and video captions docs, all read 2026-10-01.

What does LinkedIn require for the thumbnail and captions?

The thumbnail is optional, and its aspect ratio and resolution should match the video. Captions must be SRT, and the page warns to put only text in the file: custom formatting such as font colors is not supported.

How do I get a thumbnail from the clip?

Send the clip's media.sume.com URL and a time in at. The default format is jpeg, and max_edge clamps the long edge between 16 and 2160 if a file comes out too big. Leaving max_edge out keeps the source frame size, which keeps the shape matching the video. The route is unbilled and always returns 202, so poll the id.

The docs do not state a file size for the returned still, so check the result against LinkedIn's 2 MB limit before uploading.

curl -X POST https://api.sume.com/v1/video-frames \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: li-thumb-001" \
  -d '{
    "video_url": "https://media.sume.com/artifacts/artf_demo/ad.mp4",
    "at": [1.5],
    "format": "jpeg"
  }'
curl https://api.sume.com/v1/video-frames/$REQUEST_ID \
  -H "Authorization: Bearer $SUME_API_KEY"

Can Sume produce the SRT file?

The video captions page opens with "Burn captions onto an existing public video URL". Output is a captioned video, with style, language and optional script_text. That covers on-screen text, which is a different thing from the SRT upload slot. If the SRT slot is required for your placement, write the SRT yourself from your script; if burned text is acceptable, see burn captions onto video.

What about the frame rate and codec?

LinkedIn asks for less than 30 FPS. In video-trim, an exact trim re-encodes with libx264 and yuv420p, and an output.fps conform accepts 24, 25, 30 or 60 (exact only). 30 is not less than 30, so pick 24 or 25 when you conform. Details for this cap are in LinkedIn video ad specs and frame rate.