If you paint a black bar, for example, over an image with software like Paint, Photoshop, etc. then save it as a format like BMP, GIF, PNG, JPG, etc. then no one can remove it after. The same goes for cropping a photo. In both cases, you've permanently changed the pixel information inside the file.
If you paint or apply effects on a new layer in Photoshop, save it as a PSD (Photoshop native format) then someone else can disable those effects and see what is beneath. If you have an image file like BMP, GIF, PNG, JPG etc file, advanced in Photoshop and recognize the effect someone used on an image like Gaussian blur then it is possible to reverse it somewhat, but it will never be perfect.