I will try explain it as best i can in my sleep deprived state.
Create a new solid (right click the layers panel new >> solid) set the colour to black and click make compe size just to be on the safe side.
Now place this above the first image you've shown in this thread.
With the new solid selected press T to bring up its opacity control, turn this down to 0% for now, this way you can see what you're masking.
Select the pen tool from the tools panel at the top of the screen and again with the solid selected start to trace the outline of your character. (you will know if its creating a mask since the pen tools line will be yellow)
Now you can turn the opacity of the solid back up to see what happened, if the area where the character should be is black then you need to invert the mask. Again with the solid layer selected press M and this will bring up the mask, to the right of it there should be a little check box with the word 'Inverted' next to it, click that and now everything outside the character should be black.
At this point you can do a simple fade in animation by simply using the masks opacity.
If you want to apply a more fancy transition like a wipe, look in the effects and presets panel on the right, find the transition you want in there and simply drag it onto the black solid.
To animate the transition, you must have the solid layer selected, this should then allow you to see the effects control panel at the left of the screen, in here you want to animation transition completion (or something like that), to have it fade from image to black i think you need to start it on 100%.
So pick a point on your time line where you want the effect to start, press the little stop watch next to 'Transition completion' within the effects panel and set it to 100% then, move the time line along to the position you want the effect to be completed and simply turn the percentage down to 0.
Now hopefully if my explanation was clear enough and that's what you wanted, you should be good to go!
