What does "step into the picture" mean?
The most obvious and literal would be, when taking a photograph, to ask a person who is not in view of the camera's lens to step into view of the camera's lens so he will be in the picture. I can't think of any idiomatic meaning of this phrase.
This happens occasionally to professional picture hangers, when they lose their step on the ladder and put their foot through a canvas.
It's used more often as a figurative term in some contexts.
The reason you haven't received the right answer yet is that you've given no context. This is very important. Since you're a new poster, I'll ignore that, and write a more useful response.
Figuratively, "to step into the picture" means "to begin participating in something". Some examples might help:
"I was getting somewhere with Mary, until you stepped into the picture." (I was progressing well in my endeavours to court Mary, until you - an obviously superior man - appeared as a competitor for her attentions.)
"Timmy's stepfather thought he was bonding well with the child until his real father stepped back into the picture." (Similar concept to above).
"There aren't many good candidates for the election, but there's still time for talented people to step into the picture."
So that's the idiom we're talking about?until you stepped into the picture
In America that would be came into the picture.