I don't think you can from FB. I use photobucket. First I resize to about 500 then click on the last box option (img) and copy. then paste onto the post your writing.
The difficulty is putting more than one photo as the only way I have found to do this without losing the post is to open a new tab so I have two live screens - HH & photobucket.
There may be an easier way ?
Select your picture, left click the last option IMG to highlight it then right click the highlighted code to copy it then go to your post and paste the code in and hopefully it will magically appear! You'll need to open Photobucket in a separate window first