This issue is neither caused by the plugin nor specific to using the plugin.
I have looked at this in firefox 24.0 and it appears that it is normal browser behaviour to proportionally resize any image opened directly in the browser. This would be the same whether you are using the alligator popup plugin or not. If you open https://www.tefl.ie/wp-content/uploads/2013/10/coursebreakdown1.jpg in a browser window you can see that it displays the same, ie dynamically scaling the image to fit within the height (or width) of the window.
If the image was placed on a page and you linked to that page instead of the graphic, the image would be presented at its native height/width on the page and there would be a scrollbar when the image is taller than the browser window.
Having a page of text rendered as a graphic is not a good idea. Apart from hiding content from search engines it is very bad for accessibility. A better way of presenting this information would be to include the information as formatted text on a page using a landing page template. If your theme doesn’t have one as standard you can create one or commission someone to create one for you. Then rather than linking to an image of your text, use that page as the popup’s target.
