I haven't done one in a very long time, but I will be doing one for my trip that starts tomorrow through June 9th. Lots of pictures, your opinions on rides, how your experience was, what you think of the park, etc. A lot of people on here use the format of writing your trip report and putting the pics in order of your experience at the park.
Also- If you're using a phone, it looks so much better when the pics are taken sideways, aka landscape format.
To post photos in the report, I use Imgur for the picture link and then I put the link in the Img function on here. Also, type it on a computer if you can, much much quicker, and type it through a Word processing program, that way you won't lose your progress if your site crashes or something. Nothing quite as frustrating as being close to finish typing a long post through Coasterforce and then your Internet craps out and you have to start over. I have Mac so I use Pages but Microsoft Word is the same deal for Windows.