Fotoforensics is a website for advance photo analysis, you can check whether a photo has been modified or not and see embedded metadata that could contain private details, the photos can be uploaded from your PC or directly linked from a URL, there is an optional Firefox browser plugin to make image forensic analysis easier, any image that can be displayed on your browser can be analysed, the plugin gets around sites like Facebook requiring login to view a photograph.
The service supports .jpeg and .png image formats, the most common image file extensions found on the Internet, the metadata analysis can find out if a graphics editor has been used to modify the image, ACD See for example will embed the program name on the photos it saves, metadata also shows how many times the image has been edited, identity attributes and how the image was managed.
To determine if a photograph has been forged Fotoforensics will use Error Level Analysis to see the image modification percentage, the image will be saved at different compression levels and then compared with a computational algorithm to see the amount of change, this is not an 100% accurate method to detect fake photos, it is possible to defeat image computer forensics algorithms looking at high frequency decomposition by reducing colour, brightness or contrast but there are other photo attributes that can be analysed.
The website has a very detailed tutorial and FAQ explaining what results you can expect and how to interpret them, you should read it to understand what you are seeing, this is not a tool that will tell you a “Yes” or “No” answer, it is up to you to interpret the results which could turn up to be inconclusive.
You could use this tool to check that your EXIF image cleaner is working properly but do not upload anything private because the results are saved in a public URL on the server, uploading pornography is not allowed, to check if an X-rated celebrity photo is real or not you will need to find another place or they will ban your computer IP.