I suspect that, when you delete an image using the File Manager (which is the right way to do it), any references to that image (eg, in Image blocks) are also removed (as they should be). Therefore, simply re-uploading the images won't make them reappear in their original locations.

There is currently work being done on image-related stuff in the current development branch of c5:
https://github.com/concrete5/concrete5/pull/5900...
It's *possible* that these changes, when released, would allow a 5.7-to-8 upgrade to work without the need for you to delete the files. You could decide to wait and see (ie, not work on your current 8 installation), or even participate in testing the develop branch (when they sort out the upgrade aspects).

In either case, I'd recommend keeping those backups!

Thank you very much I understand that now, so the problem that I can't upgrade without deleting all the files first isn't only my problem? is this an issue in the version 8 itself? because lately I've been trying a lot of stuff to upgrade without deleting the images but nothing is working.

I haven't been following the changes to c5 closely enough to know for sure. I've just upgraded six sites from 5.7.6.13 to 8.2.1 and they all went through fine. However, I didn't have a huge number of images, nor any especially large images.

But all currently-released versions of c5 8 can mishandle image thumnailing/resizing in some circumstances. I don't know whether this affects the upgrade process but it's possible.

It would be helpful if you could identify what types of images cause upgrade problems. Possibilities are certain file types (PNG, SVG), large file sizes, or large megapixel sizes. I know that's a lot of work and probably not worthwhile.