Is it a mortal sin because I just saw it? Did I have “full consent” in the matter?
Only you know whether or not you intended to “accidentally” look at the image, or if you genuinely, for all intents and purposes, had it shoved into your face by the programming of your cell phone.
If you are not sure which of the two scenarios is true, then yes, I would bring this up in confession. Furthermore, such things need not be limited to questions of whether something was mortally or venially sinful: there are also root causes to sin that the Holy Spirit may want us to take to confession to analyze and to deal with. *Was that a mortal sin? *can be a legalistic question, and it could be that we are pricked in conscience because a behavior is merely a symptom of something more serious.
Anything having to do with lust is an incredibly grey area; and sometimes, we genuinely need advice from a confessor. Seeking advice here is fine; but we do not have the authority to make a judgment, and to effectively bind your conscience to it. A trusted priest does have such authority. Moreover, if he judges your situation as being OK, then you can have a morally sound reason to have incredibly serious doubts that you committed a mortal sin. We can speculate all we want; but our opinions
do not give you moral cetitude as the opinion of a trusted priest does.
Unless you are scrupulous, if in doubt, confess, seek advice, and follow the counsel.
… and for the record, scrupulosity involves things like being convinced that crossing the street without looking three times is absolutely, without doubt a mortal sin; it involves scenarios like being afraid that you hit someone with your car as you were driving, even though there’s absolutely zero evidence of this aside from possibly driving over a pothole.
Being unsure if you gave full consent to something is not scrupulosity; it is a sign of a conscience that needs to be formed more.