Just an update... This seems to be dependent on firmware. If I tick the "Upload to FTP/Memory Card/NAS" checkbox in the Linkage settings on my 8MP Gen1 turrets running firmware (V5.6.6 build 210625) they DO NOT save an image when an event is triggered. My new 8MP Gen2 turret running firmware (V5.7.3 build 220216) DOES save a single image when events are triggered.
On either of these models, if you tick the "Event Triggered Snapshot" checkbox in the Capture settings, you seem to get 4 snapshots saved regardless of the 'Capture Number' setting. All seems a bit hit and miss??