The actual uploading of images to the site is something I done forever ago and have next to no recollection of, but I'm pretty sure those images are only used for profile pictures and item images (in a storygame).
As such, to put an image in, you can likely link it with the rte, or just write code with the source button (which is literally just simple html tags).
Something like: <img src="picturelink" alt="Description of image, shown if image can't load and/or read by screen readers">
NOTICE: You have src="link", the quotations are super important! In this case the link is where ever you uploaded the picture.
You don't have to have an alt attribute, you can just use the image tag and give it a source. However, using an alt is highly encouraged, both for accessibility purposes and so that people know there used to be an image if something breaks.
If the RTE (which I assume you have turned on) stops you from putting your image into the thread, feel free to pm me and I can put it in for you, since I know lots of people are terrified of turning the thing off. (On the bright side, the RTE seems to be on the list of things to fix in the relatively near future, which is great news as it'll mean I don't have to bad mouth it ever again!)