Once upgraded to GarageSale 7 you need to revise your active listings so they use the new design versions which are optimized for smartphones.
Revise your running listing by selecting "Revise listing" from the "More" menu (the button with the three dots) in the toolbar.
See the help here:
The "Optimized for iPhone" checkbox is enabled by default in the "Properties" Inspector.
You can still insert your own images. You can also insert links to images but you have to add the target="_blank" attribute: