I would suggest using a tile size that is a power of two (16, 32, 64, 128...).  Than I would use a screen size that is a multiple of your screen size.

32 x 32 is a common tile size.  I would use 320 x 480 as your screen resolution (size of the iPhone screen).  That would be 10 x 15 tiles or 150 tiles on the screen at once.

if you want 100 X 50, use 16 pixels width X height.  This way you could get 96X48 and the house would be 6 by 3 tiles.

I don't want to go to deep into 2d game design but I would suggest splitting it up into tiles though.  This will allow you to define physics flags for each tile and is why the tile system is used.  For example:
 You have a text file that your program reads in that looks something like this:
1, 2, 3, 4, 5, 6,
7, 8, 9, 10, 11, 12,
13, 14, 15, 16, 17, 18,

Each number represent a 16 X 16 tile.  Your game software would load the 96X48 image and, depending on the index number, clip out the tile and draw the image at the location the index was read in.  For example reading a 1 would tell the software to load pixels 0 - 16 left to right and 0 - 16 top to down.  reading a 2 would load pixels 17 - 32...ect.

 Now, for each tile, you would define a physics flag like so:

1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1
1, 1, 0, 0, 1, 1

1 would mean the avatar could not walk through the tile.  0 would represent a door.  If done correctly, your could would recognize when the avatar is over a physics flag of zero and tell the code to change game states and render a new tile sheet that represents inside the house.

Hope that helps.

if it's jpeg you'll have to reduce the colors to get back to the original or start over. why are they zoomed?
If this is intended to be a WIP thread you should post in the WIP section.