You may be in the process of building your email campaign using a system template with a near full width image box e.g. image box is 521 x 153px and you're having a problem filling the image box width. You have tried loading in an image of 600px but it still only fills about 3/4 of the width.
1. Re-size your image to as close as ratio/proportion as you can to 521 x 153px. If you are using Paint for example you may need to remove the tick which "maintains aspect ratio" in order to re-size your image closely.
2. Check the quality of the resulting image in your email campaign, once you have uploaded your newly re-sized image to your image server list. If you're happy with the quality, fine, but if not you may need to re-size again until you you find the best balance between filling the image box and the final quality of the image.
If you need further help for this please raise a support ticket to us.