You can install our site as a web app on your iOS device by utilizing the Add to Home Screen feature in Safari. Please see this thread for more details on this.
Note: This feature may not be available in some browsers.
You don't need to use media. From a desktop, you can just drag and drop the images into the reply window.
There's also the "Upload an Image/File" button near the bottom right of the reply window which works on mobile.
How big is the file you're trying to upload? HEIC isn't a format that many browsers support. You'll want to use jpg or png.