IPT
I took a look at the help browser and it seems it is indeed not defined in Image_Processing_Tool_2. The Atoms package page also mentions that:
This toolbox provides some supplementary functions for Image Processing Algorithms
You need another toolbox to read an image.
Alternatives
I'm not sure if you are required to use that toolbox specifically. But you could also take a look at other image_processing toolboxes altogether.
SIVP
For example SIVP, which is also the only partner module in the image processing category.
Reading in an image is as simple as:
im = imread(SCI + '/contrib/sivp/images/lena.png');
imshow(im);
Help browser
The help browser in Scilab is usually pretty good and also contains examples. The toolboxes you add through the ATOMS module manager also add a section to the help browser. If you want to know which functions are implemented in a toolbox and how to use them, this is a good place to start looking.