A Timeline element can be added in the presentation editor by going to "Insert", in the upper settings menu, selecting Chart and then Timeline.

As shown, there's a bar button on top of the sequence to add items. Each item can have an image and text, just hit each of these blocks in order to edit them. You can take advantage of Slidebean's gallery of images and icons and browse them using keywords, or upload your own ones. 

In order to remove or reorganize an item in the sequence, use the buttons displayed in the right-hand side of the item when you hover over it. To move it up or down, just drag it from the arrows.

Finally, select the items distribution, to organize the timeline display and make sure to hit Done, to save the changes when you've finished.