I see MANY posts where the poster says they can't or don't know how to upload the photos.
In the Post Reply window, click on "Upload Picture".
The photo needs to be on the device you are using, although you can place a link to an external picture on the internet (as long as that pic stays in the location on the net. If it is moved or deleted, your pic will disappear). And it must not be too large (the software will resize it to a nice size, but if your picture is larger than those parameters, your going to have to use an image editing program, I use Photoshop but there are other software options, to shrink it's size. There are online sites that will do it for you). The max sizes are listed in the dialog box. See pic below. The sizes are highlighted in GREEN. Next you need to hit the "Choose File" button highlighted in ORANGE. Select your picture file. Then hit the "Send" button (also orange).
And while we are on this image, there are many posts where the picture comes in sideways or upside down. There is a way to correct this. After you hit "Send", a dialog box will appear with your pic and a link to "Insert Picture" (see 2nd photo below). If you see the picture isn't right side up, you can rotate the picture to make it right (highlighted in YELLOW). But, you have to cancel the upload and start the process over, making the rotation adjustment before hitting "Send".
NOTE (EDIT): Bobby mentioned the Gallery link in a reply below. I had no idea what it was until he posted. Here is his comment. Sounds like like a cool feature.
You may want to add to the original post, About the Gallery, And how it stores pictures for reposting on the forum.
Once you hit "Insert Picture", it will insert the code in your reply The picture itself is not visible at this point. Hit the "Preview" Button (highlighted in the pic below), and it will show exactly how your post will appear after you hit the Submit Button.
Hit "Submit".
And, NO, you cannot upload videos. You have to put the video somewhere on the internet, I use YouTube, and paste the link into your post.