All you have to do is deny you put the images there and ask them to clearly identify who did. The fact that they live on your computer's hard drive means nothing if your computer is networked.
Remember, the emphasis is for them to prove who put those images on your computer, if you don't know then you don't know, simples. 14 years working for this company is plenty to identify your character and if it's been good then that should count for a lot, otherwise it's a witch hunt which is also a question you are entitled to have answered as to why.
Regarding my own sacking, I just found out the HR manager responsible for it now works in a senior role in a government department servicing the general public. I have began a process asking whether she is a fit and proper person for the role and whether her own judgement is subject to gross discrimination as she had fabricated many lies in her written witness statement as was submitted during my court case. She aint gonna like it one bit but it serves her right - she made her own bed now she can sleep in it.