Q: How to solve this error A mixture of self hosted and EPS pictures are not allowed?
A: When listing via file exchange (non-API), images have to be hosted on the web, the image URL is then sent to eBay file exchange on the CSV when listing your items. EPS is uploading images directly on the eBay side using their uploader. They have a limit to self-hosted images, you can only have 1. And while they will allow the items to post, they won't let them be revised sometimes unless you remove all images and upload new ones directly so that they are then all uploaded through EPS (which can only be one when listing directly on eBay or when listing via API ... i.e. sku fetch + wise lister as opposed to file exchange).
As such, the developers have come up with a way for you to easily get the images needed. You'd have to locate the item on your sku fetch item list, and there's a link for product images.
When clicked, you will see a list of the product images. 
You'll be able to click on them to open in a new tab, then you can save to your PC. You'd then have to remove the current images from eBay and then upload the new ones directly through their EPS interface if they are prompting you that you need to change your images. There isn't another way around this situation when using file exchange.