This is a WordPress issue. It is actually a feature of WordPress that generates thumbnails with smaller size so that it is load faster. However, thumbnails are usually generated with fixed size, and therefore they are usually cropped.
To fix this, you can either:
Load the image in full size. You may need to change theme settings or customise theme to achieve this.
Change the existing image size so that it make the image smaller proportionally without cropping. Again, you will also need to customise the theme.
This is what I can guess from your input. It would be better if you can send a link or at least a screenshot to demonstrate the issue.