If you don't wish ID to generate a cover image for the EPUB being exported, you can choose "None" from Cover drop-down in Set up section in EPUB Export Options dialog.
If there is an image getting generated even after setting cover as None, it may be because of custom rasterisation setting being applied to one of the objects in InDesign document. The name 301.jpg signifies either a cover image or an object deliberately set to Custom Rasterise.
In case the problem still does not solve, it would be helpful if you can share your ID document (my email id: firstname.lastname@example.org) so that I can tell you which one of the items is resulting as an image in EPUB.
I think you're right. I tried exporting a fresh document and the "301.jpg" file was not present, even when I copied the export settings from the other document.
I have attached the InDesign document on your email.
Solved, I have applied a custom rasterization on the textbox of the first spread. I just unchecked "Custom Rasterization" from "Export Object Option"
Thanks a lot