Need help with strange image behaviour

Hi all -

I have an image that appears with one set of dimensions as a thumbnail, but when I open it in any number of programs, the dimensions change and some of the content is lost.
In short, when I open the image, the bottom seems cut off, the only way I can see it in full is as a thumbnail.

Note: when I change the size of the thumbnail in Finder, it will only appear in full up until a certain size.

Any info on this appreciated, as it seems like the image might look better 'un-cropped', although I have no way of seeing it.

Thanks in advance