No need to make test posts, just use the "Preview" button to see if it works.
(Just type it like this; I used a special trick to make it *not* work, so I can show it to you.)
You cannot post a picture straight from your computer, it needs to be online first. An easy way to do this is to upload your image to www.tinypic.com. (Also, I suggest you save it as ".png" (MSPaint can do this), so it will be much smaller; faster for you to upload and faster for everyone else to download.)