If you were granted commercial rights, and the person granting them had created the assets, you don't have a problem. Do you have a readme file in the download or do you remember the artist's name? You could see if he has other assets on itch.io and get his contact details from there.
If you're concerned that they were deleted due to copyright infringement i.e. the person uploading them had no rights to distribute them - you'd have to contact itch.io support with the details to see if they can help you - but you would need to know the name / package name or something that would help them identify it.
You could also do a reverse lookup: https://support.google.com/websearch/answer/1325808?co=GENIE.Platform%3DDesktop&...
This will let you know if there is a source to the images you're using or where else they can be found on the internet.