Pictures and videos are sent in full quality if they are sent over WiFi. If not, the images and videos could be compressed depending on what your cellular carrier requires.
Edit: it seems like the way to ensure sharing photos/video at original quality in iOS/macOS is to select the media in photos app, and then share, and then select “copy iCloud link”.
> When shared, photos are reduced to 2048 pixels on the long edge, except panoramic photos, which can be up to 5400 pixels wide. You can share GIFs that are 100MB or smaller.
I do not see a material advantage of iMessage over WhatsApp now.