Here's how I do it:
Reply post, then click on "Manage Attachments" below the text field.
Click on the button Add Files, then the button in the sub-window, Select Files.
Find the screen shot and "Upload" it. Click "Insert Inline" and then "Done". If it's too big, I'll get an exclamation point next to the file, and it won't add. If so, I do this:
Find the screen shot file and right-click on it, I select "Open With" and use Paint.exe. I then 'Save As' and change the file type to .jpg - this process has the effect of compressing my screenshot's file size within the parameters necessary to post in the thread. Then I go back to step one.
