Once you install Irfanview, it will associate itself with images etc. Double click on the image you want to resize and it will open in Irfanview.
Go to Image -> Resize/Resample (or CTRL-R) and it will pop open a window. You can select what resolution you want the image to be (for web posting, 640X480 is good). Select the resolution and click OK. Now, click File-> Save As. Name the new file (the old one will still be there unless you overwrite it).
Now you have a resized picture. The next task is to find a web host like
www.picturetrail.com You need to have the picture "hosted" on a web server (generally, your home computer is not set up to be a web server). Upload the file to that server. There should be a link that you can use to publish the picture such as:"httq://www.picturetrail.com/mypicture.jpg" <---not a valid link but you should get the idea.
Once you have that link, you can post a picture directly on this forum by enclosing the link between IMG tags: [IMG]httq://www.picturetrail.com/mypicture.jpg[/JMG] except the second closing tag should have an I instead of J.
Clear as mud? I had to misname the link and tags otherwise the forum will parse them and change them to a link and picture.