Some effects can be undone almost perfectly, so make sure the one you use involves a lot of randomness or actual data destruction (e.g. If you use a blur rather than a plain opaque rectangle/brush, a determined attacker could try lots of different possibilities in the image to see what text(s) get close to your image when blurred. Any spaces in the blanked region provide information about the relative lengths of the data's parts/words (mentioned in David Schwartz's comment).The height of the region could tell attackers whether the text representation of the data has ascenders or descenders (like in the letters b and p).The width of the blanked region places an upper bound on the length of the secret data.(Paint doesn't.) There are some things to watch out for, though: As mentioned in the answers to a very similar question, scribbling over part of an image will destroy the original pixels, assuming that your editor doesn't store any layers or undo history in the saved image.
