If you want to use Ruby TK for painting – and not just displaying pictures from the disk – you can use the widget TkCanvas. To this widget you can add lines, polygons, elipses and circles etc.

Now, to add a shape to a canvas in Perl you would use the syntax:

$canvas->createLine(x1, y1..., xn, yn, ?option, value, option, value, ...?)

In Ruby:
line=TkcLine.new(canvas,x1, y1..., xn, yn, ?option=> value, option=>value, ...?)

x1,y1,…,xn,yn are the coordinates of the shape. They can be changed in response to events.
It also has options such as ‘fill’, the color with which to fill the shape.
The options can be changed in response to events.

What Are Coordinates

Every shape has its coordinates, points that define the shape. You can get or configure the shape’s coordinates using the function ‘coords’.

TkcLine – points along the shape consisting of one or more lines. Each point is where the previous line ends and the next starts.

TkcPolygom – Like TkcLine, but with a line connecting the last point to the first.

TkcRectangle – x1,y1,x2,y2 are the coordinates of two opposite vertexes of a horizontal or vertical rectangle.

TkcOval – x1,y1,x2,y2 of the rectangle enclosing the ellipse or circle.

TkcArc – like TkcOval, but in addition has the options ‘start’ and ‘extent’. ‘start’ is the number of degrees from the 3 o’clock position of the oval. ‘extent’ is the size of the angular range occupied by the arc counterclockwise from the arc’s beginning point.

Learn more at http://perlhelp.web.cern.ch/PerlHelp/ (search for tk/canvas on the right side of the window) or feel free to ask your questions in a comment.