Board Thread:Support Requests - Designing Your Wiki/@comment-24034641-20131222090025/@comment-1038387-20131222131450

It's a cache issue. Sometimes Replace works fine, sometimes it takes a while to work properly.
 * Try purging the image, or wait 24 hours. These sometimes help.
 * Reupload the new image under another name (OldImage2, or something). Delete the old image. Move the image to the old name.

That last one usually helps.