Here is the Macintosh window which shows when you are in iphoto and select a picture you want to put on the TreeHouse.
First, select a picture. Then go to the top of your screen and under 'File' select 'Export'.
The window will pop up. The same window I have posted here.
With this window you can 'scale' the picture to whatever size you want.
on 'Format' select what type of image you want; JPeg etc. Then select 'Export'. It will ask you where to save the image. Tell it to save it to your desktop. Then you can bring it to the TreeHouse.
Hope this helps