Use the attachments icon, circled in red, below.
Click the
Choose File button, then browse for the images you want to upload. Upload them one at a time, by pressing the
Upload button.
Once you have them uploaded, you can edit your reply by inserting those attachments in any place you need within your reply, by clicking the little down arrow next to the attachment's icon.
A text that looks like this will appear in your reply: [ ATTACH ] 543210 [/ ATTACH ] - that's where the picture will be inserted.