Question on internal handling of images

[quasius]

Does SFML use any kind of texture compression or image re-arranging internally?
In other words, are there any performance gains by making sure images are already power of 2 sized and as full as possible with useful data and not transparent pixels?

[Laurent]

Does SFML use any kind of texture compression or image re-arranging internally?

No, absolutely nothing

are there any performance gains by making sure images are already power of 2 sized

Graphics cards are (more or less) always internally optimized for power-of-two textures, so it's always a good idea to use them if you can.

and as full as possible with useful data and not transparent pixels?

Transparent pixels are just ordinary pixels with a different alpha value, they are not "empty" pixels