Purge Cache is best used when the url is the same but the image/file content itself has changed.
Workflow in this video :
00:01 Check the Original Image & URL
00:07 Delete Original Image For Sample.jpg
00:14 Upload New File For Sample.jpg
00:29 Purge Cache Via Dashboard
00:36 Test Purge Cache Via API
01:12 Check Cache Purge
01:15 Cache Purge Is Done
Documentation
Attention
The video is recorded on an older dashboard version however the steps remain the same on the new dashboard