An easy way to make newly added images push each other forward or add on to the end of a row is to add them to a Horizontal Box container widget with the Auto spacing rather than the Fill spacing option. Also how they displace each other or add onto the end can be controlled by setting the alignment to Left, Center, or Right.
To add an image widget you can use Create Widget node then use the output pin to set which image it is, which you can get from a Data Table or an Array.