When you open a picture using IrfanView, click on the Image option at the top; then click on the "Information" option.
About halfway down, you'll see "Disk Size"; that tells you how big the picture is. If you're sending stuff through e-mail, any pictures over 30K are going to take a while to send.
Use the "Image" option, then "Resize/Resample" option to make your pictures smaller. I personally click on the "Percentage of Original" option, and then usually start experimenting with about 75%. After you change it, go back to the "Image" and "Information" to see how big it is now (you may have to close the program and then re-open it to get an accurate number).
Remember to save before closing it out.