Be sure you click to the right of the previous image before inserting another. Click "Choose File" and select the image to upload.
Do not change either the dimensions or the alignment; those are already set properly for this forum.
When you have selected the image to upload, the temptation is to click "Insert" before it has finished uploading. Be sure to wait until the top box says "Image Chosen" before you click the "Insert" button. Otherwise it will not post. When the image appears in the post box, you can again put the cursor to the right of that image to insert another the same way. You should be able to insert any number of images this way.
Any text can be typed in either before or after you insert the images.
Sometimes the text inserted after an image will appear like a link (underlined and in blue). Just highlight it and click the break-link icon (the 4th from the left) in the toolbar at the top of the box.