Assuming you have a "The Sims 2" directory and associated subdirectories in the Mac version similar to what Win users have in My Documents (I find the Mac OS filing system rather opaque):
Immediately after you capture the image for the photo or painting, the image is stored in the main Storytelling folder as
screenshot.bmp and
snapshot.bmp. These files are overwritten the next time you make a screen capture from within the game.
You can replace the image in
snapshot.bmp (
not screenshot.bmp) with one of your own (even an in-game snapshot), but (at least in Windows) you need to adjust the proportions of the image so that it looks correct on the painting or photo (the game uses a "squeezed" image that is expanded when displayed in the game). Basically, just match the dimension of your replacement image to those of the original image,
without constraining proportions. You want the image to become shorter top-to-bottom but not more narrow.
You can also use a "squeezed" image created by the game for a sim painting or photo to make a standard recolor of the
Lady on Red painting using the Photo Studio feature of SimPE. First you "normalize" the image by doubling the vertical dimension (from 450 pixels to 900 pixels in Windows) and then reduce it to 256 x 343. In this case you would save the resized image as a .png file.
Finally, if you're sim has completed a custom portrait or landscape and it's no longer available as the
snapshot.bmp file, you can retrieve the image from your sim's lot package file. These images as well as photobooth pics (at least those you've hung on the wall) are stored in the Texture Image section of the lot file, and can be exported as .png files using SimPE.
Of course, you can't do either of these last two things using a Mac, since you can't run SimPE.