Click on Manage Attachments when replying and upload your pictures.
They have to be 500x500 pixels or smaller to upload them, so you need a resizer.
I use JPEG resizer from here: http://www.virtualzone.de/
It's free (I think) and it lets you resize based on the largest dimension of the picture making it pretty simple.
Yeah, you can also use ALT-PrintScreen and get a screen shot of just the selected/highlighted/active window as well.
Then just go into paint and hit CTRL-V to paste the image in, save as a JPG and you're all set.