TL;DR: What are the best practices for making a pattern out of a picture?

Hi there! Thanks in advance for reading this, and hopefully for helping me out.

I'm here because I need help. I'm thinking of a Christmas gift for my fiancee, and I can't ask her these questions because it would give it away given the proximity. Fortunately, Reddit has a community for everything.

Anyways, my Fiancee does a lot of counted cross stitch. She always works the kits she gets at Michaels or Joanns, and then gives them away when complete. I'd like to give her the chance to make something she really likes-and something we can put up on our wall when its complete.

I've looked online and found a number of picture to cross stitch converters. I don't know the first thing about it though, so I can't tell what if anything would be better. I also don't know how many stitches per inch I should be using (amazon seems to carry all the fabric types). I still need to check local stores to see which brands of thread they carry, to see what I should choose there.

This presumes, of course that she can transition from kits to this sort of thing (again, I haven't a clue).

Anyways, the image (I still have to find a good one) is likely going to be either a leopard or a snow leopard. I'd prefer the snow leopard, but I don't know if colors matter overmuch and a leopard would be better.

The two better looking converters I've found are: http://www.picturecraftwork.com and www.pic2pat.com

Any/all advice is much appreciated. Thanks!

Edit: Also, what size is going to be best? The converters seem to be size limited, but presumably one could cut the picture in two, and get 'instructions' for each half separately?