Want to know the easiest, fastest and most fail-proof way to make a PNG? Here we go!
Quicktime:
1. Open the MP4 file that you are about to upload to us in Quicktime (this should be sized to 864x486). Scrub to the frame that you'd like to be your thumbnail.
2. Choose file -> export as
3. Select "movie to picture" in the export setting

4. Click Save
5. Open your .pct file that QT created in Preview*.
6. Choose file -> save as
7. Change format to PNG and click Save

8. A good way to test your file is by opening it in a web browser. Ensure that is is 864x486.
9. Upload to TurnHere!
Mpeg Streamclip
1. Open the MP4 file that you are about to upload to us in Mpeg Streamclip (this should be sized to 864x486). Scrub to the frame that you'd like to be your thumbnail.
2. Choose file -> export frame
3. Set frame size, quality, then format

4. Click OK and choose where you'd like to save the file to (e.g. your desktop)
5. Voilà! Test your file by opening in a web browser and upload to TurnHere!
*Preview may give you an error message:
In this case, follow the directions by going to your applications folder, right clicking on Preview, and marking "Open in 32 Bit Mode"