The maximum width of your emails and, by extension, your images is 600 pixels.
If you want an image to be narrower, 250 pixels wide for example, it is best to resize it to actually be 250 pixels wide. This reduces the file size, load time and your recipient’s data usage.
If you use Photoshop, always save your image using the ‘save for web’ option. This removes and superfluous information, reducing your file size significantly. You will see a preview of the file before you save it, so you always know what your editing does for your final image.
If you don’t have Photoshop, you can use free online tools like Kraken.io, Tinypng, PicMonkey or Aviary.