Hi Martin!
The size of the images can be adjusted here: https://i.imgur.com/N7kB6wD.png
On the same panel you can also adjust the size and the count of the rows accordin to your needs: https://i.imgur.com/oryzFUo.png
If the row max width and the image width is close to each other, they will appear closer as well.
Also, is it possible to have logos next to the headings, or is that unlikely?
I may need more details on this? Do you mean like the distance between the group heading and the logo? Adjusting the image width should help with that as well. If you get the width/height to the same ratio as the original image sizes, then the container will have less vertical/horizontal space as well.