Maybe my question is not really clear but
I don't want a picture as a logo, but I want it there if necessary
own website name text and a slogan underneath.
As is possible with many template
sorry my english is coming through google translator
yes, this is possible in many template (e.g. breez3-Standard).
But I am afraid that this is not possible with clean login template. (as far as I see there is only possible to turn on/off logo image) :/