Hi Gabor,
You are seeing broken thumbnails because the images that the Image column points to were deleted in the SharePoint site that backs your list. Microsoft Lists always stores those pictures in the list's SharePoint site. When the backing files disappear, the cell shows names like original-<guid>.jpeg and opening them returns invalidRequest.
You'll have to restore the missing image files from the site's Recycle Bin so the existing list items pick them up again, no re-upload needed.
Open your list in a browser. In the top right choose the gear icon > Open in SharePoint. If you do not see that, use the command bar menu Integrate > Open in SharePoint. Now choose Site contents > Recycle bin.
Search or sort by date to find the images. They usually have names starting with original- and end in .jpeg or .png, and they come from either Site Assets > Lists > <your list name> or from Lists > <your list name> > Attachments.
Select the files and choose Restore, then refresh your list. If you do not see them there, open the Second-stage recycle bin link on that page and check again. Restore from there if found.
If nothing shows up in either recycle bin, tell me and I will walk you through the next step.