Hi Renee Xiao,
Did you just replace the image in the design studio? If you did it will have the exact same link and the original is probably cached in your browser. Try clearing your history for the last day and refreshing the email again. You could also try using a different browser if you have multiple on your machine. I sometimes just add a version on the end of the image name like "-v1.jpg" and actually upload it as a new image. You have to then change the name in the editor as well but it means you have a history of all the different versions of the banner.
Thank you Gerard!
I did clear the history of browser and refresh the email again and again, but no change I've changed a computer now, and will try your suggestion, like change a different browser or rename the image, when the issue occurs again.