Surest way is to go to "My Trainz" above, then to "Trainz Gallery" (Note, this uses an additional login, same username and password).
Now, if you took a screenshot in Trainz and told it to upload there, it will already be there.
Otherwise, you can find your screenshot in the Screenshots folder inside your build folder, then manually upload it to the Gallery using "Add Media". Add it by drag-and-drop into the indicated square.
Once the screenshot is in Trainz Gallery, View the full-size image, then right-click the image and select "Copy image link"
Then go to your post and in the row of icons at the top, there is one that looks like a picture. Hover over it and it will say "Insert Image". Position the cursor where you want the image to be in your post, then click that icon and paste the link you copied above. Your screenshot will appear in the post.