how to load an image of 20 MB

First: if this is 20 Mb compressed file (jpg, png, tiff, etc) - then it could be really out of memory. In this case you have only one way - you have to create your own engine to work with large images part-by-part. You should be able to load small part of image, operate with it and save it to file and then load next part... Second: another possibility is described in MSDN -> Image class -> FromFile method -> Remarks: "If the file does not have a valid image format or if GDI+ does not support the pixel format of the file, this method throws an OutOfMemoryException exception."