Assuming the image is irregular and not rectangular:
In Photoshop, MASK the areas of the image to become invisible in the final image. You'll need to save it in a format that supports masking - NOT JPEG.
Once you have added the image to the Archicad Library by creating a new folder for such images and adding the folder to the library using the Library Manager, the image is available as a material texture.
Working with the Materials dialog for the Internal Engine, import the image to the material and set the size [see attached]. Enable Transparency to use the mask.
Working in OpenGL with textures visible:
Apply the material to a thin wall element exactly the extent of the image, since Archicad automatically tiles material images.
The image won't fit exactly on the wall element. Look under Design>Align3D Texture, select SELECT ORIGIN and click the bottom corner of the wall element to align the image with the wall element.
If the image is rectangular, use the Picture Object placed on the wall to call the image - no transparency, however.
Dwight Atkinson