If you have posted a message you get the quick option to edit from the pencil icon at the bottom of the message, but if you want to add ar change attached images you have to click on the green 'actions' box at the top and then select 'modify'.
On many, probably most, websites the more technical stuff is easier when using a laptop or desktop than it is with a phone or tablet but the basic functions are usually the same although sometimes the different browsers don't always support the more advanced options.