I want to post a question on how to open a certain data file in Mathematica. How do I attach the file to my post?
If your image is smaller than 2 MiB you can just attach it directly. (By pressing Ctrl+G or clicking the insert image icon)
If not, you can try any of the multitude of image hosting platforms (such as https://postimage.org/) and post a link.
The problem is how reliable these are. So to be on the safe side, it s advisable to include a smaller image in the post.
Figshare is also an interesting option, although only meant for scientific results.