Could it just be your browser cache? Have you tries ctrl + F5 to reload the page with the image?
Its not just the cache, if you do it with pngs it you can see that it replaces the file on the filesize although the image may be cached and requires a hard reload but on jpegs it doesnt replace the file at all.
I noticed this as well recently, and just now reported it as a bug.
Temporary workaround could be to first right click the image, choose Version History, and from there you can Create a new version of the file.
When i in the filebrowser try to replace a jpeg file it behaves normally (a file exists etc) but it doesn't end up being replaced.
I've tried different files and it seems its just .jpeg and .jpg that have the issues. The bug doesn't seem to be related to the installation either since I can reproduce it on episervers alloy demo site too.
Is this a known bug?