"I just saw a picture of when you were here" is better.

"I just saw a pic of you while you were here." or "I just saw a pic of your visit here." "While you were here" technically specifies the time and is incorrect, even though I know what you mean. The meaning of your sentence could be: "I have pictures that were taken while you were here."

I'm not exactly sure if I understand your question, but I think you're wanting to know how to tell someone that you were just looking at their picture right before they came into the room (or wherever you were at the time). Is this correct? If so, just say: "I just saw a picture of you" OR "I was just looking at your picture" There are a few other ways to say this but these are a good start. LMK if I'm understanding you correctly because if I'm not, I can rephrase my answer. :)