JPEGs/PNGs or other image files
Redacting bitmap images
Bitmap images are relatively easy to redact, paint over the section you need to hide and save the file.
You should be aware that some image formats (in particular TIFF and bitmap editors' native formats) support layers - the redaction painting may be placed in a layer above the original content and thus this will be recoverable. If you need to share in one of these formats make sure that the image is 'flattened' before you save it for sharing.