A website needs to provide the functionality if it is going to allow you to upload files to it. That functionality will likely be behind a security login process.
Alternatively if you have access to the website’s server you may be able to copy the file straight onto the server, and depending on its functionality your image may then appear on the website.
If you want to upload files to an Arduino system that has an internet connection, runs as a web server (publishing a website) and has an SD card for file storage then it is possible to implement functionality on your Arduino system to provide both a secure login and file upload function for files (including images) to the Arduino systems SD card. I have all of that functionality in my Arduino system that can be found at http://www.2wg.co.nz. However you won’t be able to login so you won’t be able to upload files to the Arduino’s website.
I have released the source code to my Arduino website/web server application (@ http://www.2wg.co.nz). You will find all the files you need in the PUBLIC directory of the system’s SD Card browser web page. Read the notice on the dashboard/main webpage before proceeding. Start with the Overview.pdf file.