If you're sure you republished the page on which the image appears, caching is likely to blame.
About Image Caching
Most browsers will store images for a certain period of time (which you can often change) in what's called a cache to help you more quickly view that same image later. If you change an image but don't change the image's name, the browser might not realize that the image has changed, and show you the cached image (the old one) instead.
To boost the performance of all of our member web sites, Yahoo! also caches images from your site on our servers. The server caching makes your site appear significantly faster to all of your site visitors — but like browser caching, it can also mean that occasionally updated versions of your images do not appear for you or your visitors.
How to Fix
1. If don't see your latest images, you may need to tell the cache to look for the new versions of your files, and to dump the old versions. You can reset the cache by force-reloading the page on which your image appears in your browser:
2. If dumping the cache doesn't work, and you still can't see your image, you may need to rename your image. For example, if you original image was called image.jpg, try renaming the image to image2.jpg, then readding the image to your site.
(If you're working in HTML, be sure to update the path to your image in your page code.)
Since neither the browser nor server caches will have encountered image2.jpg before, they'll automatically display the new image on your page.