messages writen over image

is there a code where I can have a button when they go to post it allows them to write their message over an image an it will show up in posts? an html code only shows up as text for this and the only codes that work are in the buttons in the message page so someone could walk me through it to make a button that allows this code in messages?

How do I use a table? what is it written in english? Where do I put the code, in the templates?