One of the quirks of T4, is that the Media Library allows editors to upload files with the same filename in the same Media Library folder. So if there are two images named photo.jpg in the same folder, then either one could be shown on the website as the website won't know which one to show. This is particularly an issue with images of people, as the wrong person's image may display.
The most common cause of this is when files are uploaded using default or generic filenames e.g. image1.jpg, photo.png etc.
When uploading an image to the Media Library, please try to ensure the filename is as unique as possible.
A few suggestions
Image of a person
You could use their name followed by the date e.g. john-smith-170325.jpg or their initials js-170325.jpg
A research-related image
you could include the research subject or the associated lab followed by the date e.g. hydrodynamics-laboratory-wave-tanks-170325.jpg
Academic event image
You could use the subject followed by the date e.g. software-engineering-design-lecture-170325.jpg
You do not have to append the date to the filename, but this is an easy way to mitigate the risk of filename conflicts.
If you need any further advice, please contact your Faculty Web Officer.