If you look at the various different sizes WP creates in your gallery can you find one in the correct size?

If not, consider opening the image in an image editor on your PC and using Save As to create a new version with the desired dimensions. Although you can use HTML attributes to squeeze the large image into a small space, this will slow down page loading compared with sending an image that's already the correct size.

If you don't have an image editing program, Microsoft's free Photo Gallery (part of Essentials) would be one option: http://windows.microsoft.com/en-us/windows-live/photo-gallery

It's the web site is using over-sized pictures (some do it on purpose).

Use Your zoom controls.

<Control> + (plus) enlarge <Control> - (minus) reduce <Control> 0 (zero) restore