With the specification images the current limit is 500kb per image, if you have images beyond that they'll need to be made smaller before loading.
The easiest way to do this is using an online image compressor. Although there are several options out there we've added a link below to the one we suggest:
To use this compressor is quite easy, load the files in the top, set the shrink settings, and hit start, then download the smaller files its produced.
How much you shrink the image is up to you but we find that the below settings generally shrink below the 500kb mark from an average large image.
Select smallest file
Select "Resize for email"