Blend? Merge? Image seamlessly with background

Hi, I am sure this is a common question, but because I am a newbie and not familiar with terminology I can t find the answer. I have to change background to some images, I found the tutorial for that, but I need the transition between image and new background to be as seamless as possible. the image is processed with a watercolor filter, so the margins are very blurry, how could I go about doing this? Do you think it s better if image one has a white background originally, or a background of a color that is similar to that of the final background? For example imagine these two pictures in a dark purple background. Ps I already tried to apply the watercolor filter to both image and background, but the background colors come out completely different from what I need. It s not a photoshop watercolor filter so I can t choose which part of the image to process, or export a transparent gif. Thank you for any advice! 
