There were numerous updates, here are only the more 
important things.

Version 1.70
- Viewing TIFFs (uncompressed, LZW, packbits, Huffmann
  fax group 3, fax group 4)

Version 1.80
- Viewing PCX and RLE
- Viewing grayscale TIFF
- Fixed some bugs with GIF

Version 1.85
- Windows 95-dialogs (open/save file)
- Viewing 2 bits per pixel PNG
- Saving file as TIFF

Version 1.90
- Viewing Photo CD
- "Fast directory view" (jump to next image in the
  current directory with space, to previous with
  backspace)
- Set image as wallpaper

Version 1.95
- Viewing TGA
- Image flip/rotate
- Image copy/move to another directory
- New tab-dialogs (Menu: Options->Properties)
- IrfanView32 doesn't support Win32s any more

Version 1.97
- Viewing RAS
- Fixed some bugs with fast directory view and GIF

Version 1.98
- Viewing ICO 
- Zooming in and out 
   in: 150, 200, 250, 300, 500, 600, 700, 800, 
   900, 1000 %
   out: 90, 80, 70, 60, 50, 40, 30, 20, 10, 5 %
   (known bug: zooming in large images works till
    now only up to 500 %)
- Resize image

Version 1.98a
- Fixed bug with 256-colour-ICO
- Fixed bugs with fit-to-desktop and resize
- New menu outfit
- Zoom percentage shown in title bar
- Positioning window somewhere else than top left 
  corner
  (biggest known bug: IrfanView still can NOT correctly
   resize images whose dimensions exceed actual screen 
   resolution; if you do not want to cut edges of your 
   image, void saving in this situation)
