As Shawn said, a good way is to use the triumphRAT.net photo album. This example uses that.
First upload your pictures to your triumphRAT.net album (I am assuming you already know how to do that).
Have two windows open - the one in which you are making your triumphRAT.net post, and another one with the picture open.
Select the picture you want use. Underneath the picture is a column of icons:
There's a final icon underneath the "linked thumbnail" line at the bottom. Click on that, and you get two extra lines of code:
You can grab hold of the bottom line of code, and drag it (or copy and paste it) into the post that you are making. Your post will now have that line of code, something like this:
Now submit your post, and hey presto, as if by magic the picture appears: