what I want to do is a image recognition for a simple app:

given image (500 x 500) pxs ( 1 color background )
the image will have only 1 geometric figure (triangle or square or smaleyface :) ) of (50x50) pxs.
python will do the recognition of the figure and display what geometric figure is.

any links? any hints? any API? thxs :)
If you know the statespace of your data, you can use Principal Component Analysis. With PCA all of the objects must be posed (in the center of the screen). PCA will not do detection, but it will seperate objects into unique layers in which you can identify as being a triangle, etc. Also note: this is not scale or rotation invariant. 
[I can't remember what this technique is called, but its similar to how the postoffice does handwritting rec]
If you can handle only non-curved curvfaces, you could do edge detection, and then do sampling at intersections to get an approximation of similarity.
python image recognition