By default, WordPress will automatically generate a few different sizes of images. You can adjust the sizes here: Dashboard -> Settings -> Media -> Image Sizes
.
If you want to prevent certain image sizes from generating upon your uploaded images, you could use the filter intermediate_image_sizes
to remove the generation of certain image sizes.
Example
function mbe_filter_image_sizes($sizes){
unset($sizes['thumbnail']);
unset($sizes['medium']);
unset($sizes['large']);
return $sizes;
}
add_filter('intermediate_image_sizes', 'mbe_filter_image_sizes');
If you wanted to add your own image sizes you could do so using the function add_image_size();
Example :: Fixed Width and Unlimited Height
add_image_size('f_width_u_height', 620, 9999);
Example :: Fixed Height and Unlimited Width
add_image_size('f_height_u_width, 9999, 620);
Example :: Fixed Width and Fixed Height
add_image_size('f_width_f_height, 620, 620, true);
Note: If you want the changes to reflect upon existing images, you will need to either re-upload the images or, use a plugin.