If you know what the person looks like then be sure to utilize visualization in your spell. A personal object can replace the picture just remember to focus on the image you can conjure up in your head as well.
Just my own opinion, but maybe just let the person be. I guess I just don't understand why some people want to do things against anothers will. Like forcing someone to think about you non stop. Maybe it just wasn't meant to be that way or maybe you haven't waited long enough for the spell to work. Some spells do take time before they decide to work.
Mary, Magick will only work if you fully believe it will work. Asking how long it will take is very impatiant. If you will it to work it will. Find a different spell, one that best fits your needs. Do this spell with a full heart and KNOW that it will work. If you do this and never doubt it (not even alittle) then your spell will work.
I also think that you should not perform a spell that makes a person do or think something against their will. Don't get me wrong, I've had my temptations to do this before. You just have to think through it rationally and try a less controlling method. For example, there is a person in my school that I want to notice me and I serioualy considered using a spell but then decided that I should just talk to thm more and make myself seem more open to them. Just try that first and think rationally about controlling another thoughts or actions and how it would feel if someone did that to you. I'm guessing you would hate and at least you would have the leberty of understanding what was happening, the other person who is unaware of magick will have no idea and freak out. Just please consider this.