Usually when I copy-paste stuff into my image, I don't use selection at all.
I copy and paste the entire image, then I use the layers to hide the part of the image that I don't want to be seen. I just color the alpha channel black.
Around the edges, I take a smaller, fuzzy tool that doesn't have a clearly defined edge to color the alpha channel. That way if you color a little bit of your picture it doesn't matter - it's not erased completely, it's just slightly transparent which often works to blend the picture into the surroundings...
I copy and paste the entire image, then I use the layers to hide the part of the image that I don't want to be seen. I just color the alpha channel black.
Around the edges, I take a smaller, fuzzy tool that doesn't have a clearly defined edge to color the alpha channel. That way if you color a little bit of your picture it doesn't matter - it's not erased completely, it's just slightly transparent which often works to blend the picture into the surroundings...