It's always better to upload an image with your desired size to avoid distortion. Please consider resizing images in the editor a last resort.
Although you have the option of resizing your images in our email text editor, each email viewer displays resized images differently at times, causing images to be distorted.
As such, we advise you to upload an image with your desired size. This should be a size of image that works for mobile viewers, and is not a significant degradation from the desktop email viewing experience.
For guidance on the recommended image sizes for particular areas of the product, please reference our help article on Creative & Image Guidelines.
For a list of free image resizers, one option is to reference this resource on Image Resizers.
In addition, it's always worth being aware that resizing raster images (.jpg, .png) can introduce some level of blurriness - if your image is actually a graphic that starts as a vector image (.eps, .svg), we recommend going to the original vector image and exporting it at the exact size you want, instead of resizing an already exported raster image version.