Hello Annika. From the information you gave us it seems like the problem might be one of two things:

• On top of the MB limit, there is a pixel size limit too. Your files cannot be more than 9050px on either side.
• Even if the file is under that size and under 16MB, if the internet connection is slow and uploading that single file takes more than 30 seconds the site can time out and cancel the upload. You can try reducing the file size a bit or getting closer to your router if you are on a wifi connection.

Hope this helps!