I can't find any way to delete a file once it has been uploaded. You can delete the file from the HTML editor but the file is still in the Repository. There seems to be no delete option in the File Picker.
Also, if you upload an image then modify or update the image, you have to change the name of the image to see it in your editor. I discovered that this is a cashe issue and if you dump your cashe, then the new version (under the same name) will be displayed. Is there a way to automatically refresh the cashe when you upload an edited file to the Repository. This would avoid a lot of confussion and frustration.