Hi Harold,
The forum will support 1800x1800 pixel sizes - which is, quite frankly "huge". To "resize" your images to 1800 pixels on the longest side, you can use a number of various tools - and most are free.
Lets try this:
http://lmgtfy.com/?q=How+do+I+resize+my+picture%3F
Let me know where it doesn't work, OK?
John