I have not tried to modify any image after uploading.
Using a Mac, here's my routine.
Take the pic and transfer from camera to computer. The image manipulation software that came with the camera is limited, and quite clumsy, so I don't use it. The Mac comes with an application called "Preview," which is can easily do simple manipulation. Rotate, flip, crop, resize, rudimentary color correction, save in various image formats. I also have have Photoshop Elements, and a couple of free-ware editors. I haven't used a Windows machine in some years, but I know there were several free of very inexpensive image processing programs out there.
Conceivably, there are online image manipulators also, I've not looked.
Usually I crop the image, and save it in a smaller size to make the file quicker for transfer. Then I save to the "desktop."
The I hit the "more reply option" button. Click on "Choose files..." from the Attach Files panel. Navigate thru the file requestor to the "desktop," if it is not already at that directory, and click on the desired image(s). When uploaded, click on the "Add to post" that is the option to the right of the upload thumbnail that shows up in the Attach Files pane. The image is inserted after the cursor.
Then "Add Reply."
I don't see that there are any options to edit the image within the forum post editor.
Hope this helps.