Files Have No Compression When Zipped

I'm trying to compress an image file that is 14.7 MB in size, but the compression programs I've used all keep the file at the same size after it has become a zip file. The options I've tried include the "compress" option built into the Mac OS X, as well as WinZip for Mac.

When I check the Get Info window for the original and zipped file, they both indicate the same file size: 14.7 MB. I've also tried compressing larger files and the results are similar - compression on the zip file is little if any.

Why is this happening and how can I actually compress my image file?
Thanks for any tips.

What is the file format of the image file?

if it's jpg, it's already compressed.

Not everything compresses to a smaller size..text files compress REALLY well..but a lot of other formats are already in their compressed form and will not compress any further, in fact the resulting ZIP file might actually be bigger than the original file.

Why bother compressing a single 14.7MB file? If you're trying to fit it as an attachment in e-mail or something, you might want to split the file instead to make it work..

The image is in jpg format, so it sounds like its already in its lowest level of compression. Thanks.