You could either go to the Community Menu > Gallery and upload there. Then you can use the Image Menu on the forum post to insert them. Or you can directly upload them using the same Image Menu. They'll still go into your Gallery, but in a general Upload folder, while the first method lets you organize them a little better into your own folders.
Best to not use full res image right from a camera. May work, but sometimes the sizes of them choke the server.