Having trouble cropping an image...help? (Details follow)

Hello…I don’t have Adobe Photoshop at the moment. I just need basic cropping occasionally and I am using Lunapic. I am attempting to crop a picture but am having trouble.

The photo is large. So, it is larger than my screen. The part I want to crop is also larger than the area visible on my screen. If you drag a cropping box, you cannot scroll down to get all the area, because the cropping box disappears or goes wonky.

How can I zoom out to get a smaller image on my screen to crop a picture——without shrinking the size of the original? In other words, just zoom out to see the full area to crop on the screen without affecting the original size? Is there a zoom in and zoom out feature on my computer/program that I can use? I use Firefox (not 4.o which is awful, the old one) and Windows 7.

Or if there is some other way…to get a crop box to work on an oversized photo (that is bigger than your screen) please let me know.

Thank you!