Flightster
1. Click "Post Reply";
2. Scroll down to "Attach file / image";
3. Click "Browse...";
4. Find the screenshot you want and double click on it, you should now see the directory location for the screenshot to the left of the "Browse..." button;
5. Click "Reply To Thread" to post the screenshot.
You can post several screenshots at one time by clicking on the "Add another Attachment" button and repeating steps 1-4.
This will automatically resize your screens to what the site needs it to be and won't mess up the layout