Upload your image file to image hosting site, copy the Direct
link and paste it into the bar which pops up after clicking 'Insert Image' button - the fifth from the right (or the sixth, if you count smiley).
I recommend using postimages.org, which does not require to set up an account and has "No expiration" option. Proceeding as described above, you don't have to resize large images (resizing is done automatically).
The button for inserting a link: