I only use NTF files in Map Info or sometimes ArcGIS and off the top of my head I'm not certain that they can be 'converted' to .jpg or similar. It may or not be of help but it can be converted to be used in CAD systems such as AutoCAD.
The most obvious suggestion (assuming you can view the files) would be to do a screen dump (by pressing print screen) and pasting into a graphics package. Then just crop the file to get the image you require.
You should be able to download a NTF viewer - a quick google search will confirm this.