The FMF forum software offers different ways to reply to a post.
Option 1The easiest and most commonly used is the simple reply button, this one:

This one is found below the bottom-most post and it is used when you simply want to reply to a post.
Option 2The second option, also used quite frequently is the Quote Post button, this one:

Slightly confusingly, it's also labelled reply. It is located at the bottom of each post and it does create a reply but it does so by quoting the post in which you click this button.
A common mistake made by new members is to use this one to reply to posts in View. The reply will include the pictures from the original post and quoting pictures is not allowed. Either use the simple reply button or remove the URL(s) to the image(s) from the post you're quoting
Option 3The last and least frequently used option is the multi quote button, this one:

In a thread containing several posts that you want to quote, it is used to select those posts.
When reading through a thread, you can select multiple posts to quote. The buttons in the selected posts will change into:

When you've selected all the posts you want to quote, click the simple reply button

and the text of the posts you selected will already be inserted in your reply
Merged RepliesIf you reply to your own posts within a certain, short time period, the replies will be merged with your original post.