Well, I was worried I was going to look at Jesus sexually, so I told my eyes not to look when I saw another picture of Jesus that way, but my eyes did look. That said:
- Is this a form of sin or mortal sin?
Only if you freely and intentionally looked for the purpose of stirring up lust, and it certainly doesn't sound like you did, so don't worry.
These thoughts are probably demonic in origin, to try to get you to avoid meditating on the cause of our salvation, so when this happens, try saying, out loud,
"Spirit of Lust, I command you in the name of Jesus to leave me and go to the foot of the Cross and receive your sentence.",
and see if the thought goes away. Or try,
"Get thee behind me, satan."
If that consistently does not work, find an image that is less sexually arousing,
e.g., the Cross of San Damiano or something like that.