Squares don’t show up when trying to resize pictures

Hi.

I’m having a problem with the squares not showing up to resize my pictures when I try to add it to my blog post. It worked fine on my PC but now I’m using my dad’s mac (I can’t get internet any longer on my laptop) and the squares don’t show up. I’m not sure if that’s the cause of the problem but that’s the only difference I can think of. Any idea how to get them to appear again?

Thanks.