The feed icons are checked regularly (I think it’s every 12 hours? I don’t know). So you could delete the files in feed-icons and TT-RSS will re-add them. However, if you know you have exactly two that don’t match you can just delete those two or manually set them.
To delete them, just click on the feed in question and note the URL. You should see something like
f=12 in the URL. That number is the feed ID, so delete the corresponding
Or, right-click on the feed icon, select Edit, and one of those tabs lets you manually upload your own image for the icon.