How can I place a whole photo at the top of the home page of my groups?
I know there is a video about this, but that's not very suitable for my
not so fast connection!

Oops, sorry

I've followed the directions for placing a part
of a photo in the top banner, but I want a whole image instead. Is it
possible ?

Depends on what you are after.

If you want a photo to appear across the whole or part of the banner, you do need to follow the instructions here. You will need to edit the photo so you have two, one 98px and the other 78px high. To make sure you cover the resolution of pretty much any screen that will view it, make it 1600px wide.

There are are a number of good image editing programs out there, some you have to pay for (which can be easier to use) and some which are free, but perhaps less intuitive. I use Gimp (which is free), which does let you scale images. Steve uses PhotoShop (which we had to pay for!).

On the other hand, if your question is around how you get a photo into the main page, that's a different issue, and you us Inline Knowledge entries for that. The video does indeed cover it, but I have now written a knowledge entry here

All the best

Nigel