In the theme, for the featured image there are predefined image sizes so that all your images are uniformly cropped.
Please use the images of the following sizes (in px) with Blossom Studio theme.
- Blossom Studio Slider: 527 x 702
- Blossom Spa Single post: 1170 x 878
- Blossom Studio Featured Area Section: 259 x 346
- Blossom Studio Blog Section: 765 x 574
- Blossom Studio Shop Section: 371 x 496