Either upload them via the "attachment" control panel which you can see if you click "post reply" or "go advanced."
OR upload them to a 3rd party host such as photobucket or imageshack, copy the URL, and hit the button that looks like a mountain and sun. Or you can copy the URL and put [ IMG] in front of it and [/IMG ] behind it (except omit the spaces).
Then that means you need to edit it down and use better compression, which can be difficult and/or frustrating to do.
Sign up at imageshack.us or photobucket, or flickr, for free, and host your images there. Most forums limit the attachment size because attachments consume server storage space and bandwidth.
On my computer when I view the pic I can hit the email button and a screen comes up and asks me if I want to resize the pic to small or medium for the email. I choose the medium and it comes up as an attachemnt in an open new email. Then I open the attachement pic and save it under a new name. Then I can upload the smaller pic. Hope this helps.