I also have Irfanview. To resize my pictures, I go into File at the top tool bar, open a file, Click on Image in the tool bar and choose Resize/Resample. On the top right of the screen there is a box with some standard sizes. You can pick one. I usually click twice on Half. This is a good size for e-mails.

You can also use it to custom size, by playing with the width and height.

Mary