As written, this is borderline off-topic. But the answer could apply to photography as well, for example, if taking pictures in a war zone where recognizable faces could be dangerous for the subject.

To answer it, we have to look at two things.

First, the way the information is stored in an image file. Generally, each pixel is simply represented by triplet of values, for red, green, and blue. Mixed together, this makes the color. If you change the value of pixels in a certain area to 0, you black them out, and the original information is gone. So, that's pretty safe -- if that's all your image contains. Many image formats also include invisible metadata -- information about the camera the photo was taken with and possibly things like location tags. Those need to be dealt with separately. Additionally, some image formats retain multiple layers and may even include undo history -- this includes Photoshop's PSD files and Gimp's XCF files. (And possibly some TIF files.) If you export to PNG or JPEG, you are probably okay.

Second, consider the way in which you redact your photo. If you use a drawing tool and replace the pixels as above, this is fairly safe, because you are adding new information and destroying the old. If you use some sort of mosaic or blur filter which takes the existing pixels and transforms them, it's actually possible that a clever reversal of the algorithm could get back more information that seems possible. So, don't do that.

If you're confident that you've masked the pixels using a safe approach, but a little unsure if the file itself is safely "cleaned", take a look at What tools exist to remove metadata from photos?, which gives several good approaches. (Personally, I use jhead -purejpg.)