On the amp docs https://www.ampproject.org/docs/reference/validation_errors
it says that source requires a media tag (amp-audio, amp-video, etc.).
I have a picture tag with multiple source elements but when I try to transform it to amp-img I get the error that it needs to be amp-video.
Do you know why it doesn't accept amp-img as its parent element?
Is it not considered to be a media tag.
I think you are trying to define different image URLs for certain resolutions/sizes. Have a look at the srcset attribute of <amp-img>.
There's an example here: