Does the source image need to be in some raw format ?
Or a hi-res JPEG or HEIC is fine?
I don’t think, there is a real rule. RAW gives the most room for editing, but i would be astonished, if anybody would see a RAW as obligatory.
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A heic or jpeg gives so much less room for editing. Raw is preferred, but indeed there is no rule.
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It’s called Play Raw not Play Jpg. 
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As people have said it would be acceptable for plawraw but editing is much more limited. I strongly recommend shooting RAW for images that you want to edit. I often shot RAW plus JPG, but then delete the JPG once the RAW is edited. So I even wonder why I shoot JPG ever.
I’ve been shooting jpegs quite a lot before and I’d say that if you’re fine with the way your camera processes the images, you can just shoot with a reduced contrast preset and then edit the jpeg. The problem with shooting jpegs is that some scenes are inherently more contrasty while others are inherently more blank, and there’s no camera manufacturer that provides a convenient and quick way of controlling the contrast on-site at the time of shooting. You can make it less of an issue by setting the jpeg profile to be low contrast, because adding contrast is less destructive (and therefore demanding of the “pixels”) than removing contrast (you can’t recover data that isn’t there when dark things get brighter)
RAWs have more dynamic range than pre-cooked images like jpegs. The RAW gives the users more of a clean slate to demonstrate how they go from RAW to jpeg. It is for all intents and purposes, the name of the game. If you’ve got nice jpegs, then you would want to Showcase those images.