You need to use the same code that the "Insert Image" button on blog posts creates. Here's a picture of that code to refer to - I can't add it as text, so don't try to copy/paste this:

The words between the square brackets [ ] are the label for the image and the bit between the round brackets ( ) is the path to the image.

You can find out the path to an image by right clicking on it and selecting properties from the pop up menu. Look for the Location item on the window that pops up. For files here on Cre8Buzz, highlight from "/user_files" to the end of the file name, missing off the bits after .jpg

Here it is in action....

And here's one from my gallery on my own website (not on Cre8buzz) - you have to include the full path