You need to upload the EPS with the JPG at the same time. SS suggests a 800x800 thumbnail, but I am going full res 3000x3000.
Just because for some agencies you dont need to upload the JPG, for some agencies you can choose to upload the JPG and for some agencies you are allowed to upload the JPG separately as individual illustration.
Anyway, make sure the EPS and JPG are uploaded at the same time and that the files names are identical except for the file extension.
Filename00001.jpg
Filename00001.eps