At first, make a custom block. Right click it and select "Edit block definition" from the dropdown. Now, write a comment and attach it to the block definition hat block. If you change the size of the comment, the size of the help screen will be changed, too. Currently, you can't add images, but if you include a URL starting with "http://" or "https://", it can be clicked in the help window and it will be opened in a new tab.