When you’ve finished your post click on the upload file button and select your image, then post as normal.
Both work in my case, android phone or Firefox browser on the computer. Also if you go to the "more options" editor, you can even copy the file and paste it into the browser window, and it uploads the picture. If you decide to take a 200 megapixel picture on your phone because you can, you have to use the picture editor on the phone to make it lower resolution and smaller size before uploading. I never use tapatalk because it annoys me in general, each time I thought "give it a go again", thats followed by "meh, why bother".. [TheSamba has a much better pictures interface featuring your gallery so you dont have to re upload pictures every time you want to use them, and you can upload multiple files with descriptions so other people can find "picture of a Siberian Hamster running the engine of a 1954 barn door bus"]