I have InProgress.png in my Site Assets documents library (SharePoint 2013). I want to replace it by new version of this picture with this same name (it's important because my workflows are using this picture and if I change it's name workflows will not find it). I tried to upload new version to Site Assets by checking Overwrite existing file option. SharePoint uploads file but didn't change a picture (there was new date in Created field but picture was old). Then I deleted old picture, uploaded new one (called InProgress.png), SharePoint done it but picture didn't change again. Then I uploaded new picture with different name (InProgress2.png). In this case SharePoint correct recognized my new picture, so I deleted old one and changed name of new one to InProgress.png. But after that, without uploading anything picture returned to old state.
It seems that SharePoint can read my InProgress.png picture only in one way. Maybe SharePoint is holding this file and it's name somewhere in memory and do not allow to overwrite it?