This can happen for a number of different reasons.

1. The filesize, and/or dimensions of the image is too big. Trying to upload a 2MB with the dimensions of 1575 x 2175 (w x h) is much too big. The average width of an image should be no more than 800 px wide, and the filesize should be around 200 k.

Remember, the larger the filesize of the image, the longer your subscribers will have to wait for that image to load up within the e-mail.

2. The image filenames are are incorrect.

Trying to upload an image like this: (my image.JPG) will not work for two reasons.