Hi,
Drag any file from somewhere like the Desktop or Finder window to the text entry field in Messages.
It will show as a highlighted item.
Click the space bar or add text then Send
Pictures sent this way should display In line.
Making the main Messages window bigger or smaller will have some scaling effect.
(iChat used to go as big as the window could be made)
You can also drag direct from iPhoto (if you have the Screen Space to see both side by side)
In some Apps you can highlight a pic and then chose the Share option.
This then gives you a list of methods to send it
For Messages it shows you a dialogue box with the item already place in it.
You then have to choose who to send it to.
From Preview.