Welcome to the Flarum community ?
When you have specific questions please use the search box in the top right first. Questions like these are asked quite often and repeating their answer is often more bothersome than not replying ?
On these forums uploading files has been disabled, you can - however - use the bbcode [img] or the markdown code  to show a remote image.
For your own Flarum installation you can look at the extension flagrow/upload, installable using composer require flagrow/upload.
Hope this helps!
