image header rotation: How to delete an uploaded image ?

I just create a new blog with twenty eleven. I uploaded 3 images for headings and see that the image rotation was doing great. But I was not wanting to keep these images and then: horror ! no way to remove these images which where not great images (not great enough for a photo blog!)! So I remove the images directly from the upload folder and upload the new good images. But now there are no images at all on the header half of the time. I saw too late that the images are uploaded at the end of an array and that the 3 first values of the image array are now empty!

What can be done to correct that ?
Can I change theme and then change to twenty eleven again to reinitialize the stuff .

I can go in the code if needed !