Look for and click
When using the manage attachments page, ignore maximum file size if uploading an image from your computer's drive. You can also attach by inserting an image url on the manage attachments screen but that will only work if the file size is less than 100kb.
Take a look through the forum FAQ help section posted above by clicking the FAQ button. The basics and more are pretty well covered. If using the little landscape button to use img /img tags

, try to limit width size of images to 800 dpi, so as to not make older monitors have to scroll sideways a bunch.
Click reply to start and not the quick reply window.
Cordially
Glen