I use a Note 4, which I'm on right now just to see how it's done. I usually upload to Photobucket but I also use a PC a lot.
I'm currently using Chrome as a browser.
First you need to be in "full editor mode" on the forum, as opposed to "quick reply". There's a button by the text box you'll see. Then once you get in the page that has the full editor, down below the text field you'll see two " tabs" where the first one says Options and the second one says Attachments, and you want the second one. Then click the Add Files button, and that'll open up a window from your phone that gives you options for either taking a pic, video, or going through your documents. Click documents, and that'll take you to another page where you can go down to Gallery and that takes you to your phone pics album.
Now as you guessed, file size is an issue. So if your camera is set to take max resolution pics, you'll need to edit them in your phone to a smaller size before you upload. You could also get a Photobucket account or similar pic hosting account, and upload your pics there to full resolution, and then copy them into your post.