Using a PC or laptop:
Under the box where you type your reply there is another box which you can 'drag and drop' picture files into (file must be in one of the formats listed underneath that box.)
To Drag and Drop - find the photo you want to upload in an explorer window, select it by clicking on it and keep the mouse key held down as you drag it to the box under your reply. Google or search Youtube for "how to drag and drop" - this will bring up videos for various operating systems.
An alternative way is to click on 'choose file' which will take you to the list of files on your computer, where you can find and then choose which one you want to add.
You can add further pictures if you want to (can't remember how many offhand but it won't let you load above the preset limit per post.)
Click 'Post' to complete the process.
Using a smartphone:
If the picture/s you want to post are located on your smartphone, use the 'choose file' option as above, which will take you to a screen showing where all the pictures on your phone are located.
Click on the desired location, then your chosen picture, then click 'Open' (or ‘Done’); this will add the photo to your forum post (the filename and thumbnail image will appear below the 'Cancel' button.)
Click 'Post' to submit your text and picture.
You can click 'Preview' to check your post first, but note that your picture will not be visible in the preview using either method, even if successfully uploaded.
The issue of pictures displaying sideways is complex, and there are many workarounds, all a bit too complicated for this 'keep it simple' guide.
Best way of avoiding the problem is to always take your pictures in landscape, rather than portrait mode.
You don't have to worry about trying to change the size of the picture - the software resizes it for you unless the file size is greater than 5MB, in which case you will need to use photo editing software to reduce the size, or re-shoot in lower resolution.