There's a few different reasons this could be occurring:
Make sure you are using Google Chrome - you may be unable to upload your photos because you're using an older browser like Internet Explorer.
Your cache or browser history could be full. Regular cache clearing helps keep sites upload speeds maximised and limits buffering, if you haven't done one in a while do a cache/browser history clear, close your browser down and open up in a fresh new window.
Make sure the photos are in the right file format - Bluethumb can only accept .jpeg, .jpg and .png files. For example iPhone's default to HEIC file format for images taken with these devises, our site does not support this file type.
Ensure each image size does not exceed 5MB and/or 2500px wide. This is normally the main reason for failed image uploads. Check your file size and resave at a lower size if needed
Check the image meets the minimum file size to upload, the minimum file size is 100KB and or 1000px width.
Ensure you have listed all the mandatory images required, you will not be able to publish a listing if "Primary Photo" and "Artworks Only For Previews" images are missing. These are the minimum images needed to complete a listing (we obviously recommend listing more).
We strongly recommend artists use desktops/laptops for account management rather than mobiles, the mobil experience has specifically been designed for buyers and some of the artist functionality is not optimised on these devises.