(This is based on Pages version 4.3 / iWork '09.)
It’s perfectly possible. When you create a new Pages document, you can choose between two different types: A Word Processing document or a Page Layout document. You are probably using a word processing document currently as this is the easiest type to start with: it has a fixed central text area on the page where you can directly start to type your text. However, when adding images, shapes, lines, text boxes, etc. these objects are automatically placed inline to flow with the text. Although each object can easily be switched to not flow with the text (‘Floating’ button in the top Format bar), it’s still impractical for composing illustrations with many separate elements.
To create the illustration, you should therefore start a new blank Page Layout document. These have no default text area, so you can freely place and arrange all objects on the canvas.
Place the screenshot image in the middle and start adding text boxes and lines, and position and fine tune everything. When finished, select all elements and group them (
Arrange > Group), then copy-paste the group into the main document like an image, and configure the text wrap via the Inspector window.
The main tools you use are: Text Box, Draw tool (pen from the Shapes menu), alignment features, and the fine settings done in the Inspector window. The pen tool is the one to use for multi-angle lines (and Bézier curves, but those are not relevant here). I’ll just give some tips:
• Click on Text Box in the tool bar to add a text box that you can adjust in size and position freely. Type a text, adjust the font settings, then make multiple copies for the other text bits by holding down the Option key and clicking+dragging the box at the border. (You could as well apply predefined text styles if you want.)
• When placing the text bits, see to it that all text left and right of the image share a common ‘baseline grid’ (see blue guide in the screenshot). One way to achieve that might be to place only two large text boxes (left and right of the image) which you keep aligned at their top borders, so that text lines in both boxes automatically share the same baseline.
• The Draw/Pen tool may take some practice: choose the tool from the Shapes menu in the tool bar (or press Cmd-Option-Shift-P), click somewhere to set the first anchor point, move the cursor to the next position and click again to place another point, and so on. Keep holding down the Shift key during this in order to keep the line segments straight and constrain direction changes to 90° angles. When a line is done, press the Esc key to exit the line drawing mode, then use the Graphics Inspector to set the stroke width etc. You can adjust the positions of anchor points afterwards by clicking twice on the path, then clicking on an anchor point to select it (hold Shift key down to select multiple points) and moving them with the arrow keys or the mouse. Press Esc again to end drawing mode.
• When the black lines run into darker areas of the image and become difficult to spot, try giving the lines a white shadow to let them stand out from the background. (Select the line, and in the Graphic Inspector, check Shadow, set the colour to white, set 'Offset' to 0 pt and 'Blur' to 5 pt or so.)
• To align multiple objects at the same border, select them and choose
Arrange > Align Objects … (or use the right-click menu to choose alignment options.