To read a PGM file in LabVIEW, you need to use the Read Binary File.vi.
The header format is as described below:P5
<maximum grey value>
For an 8-bit image of size 320x240 the header would look like the following:P5
This header is followed by an array of pixel values corresponding to the image. If the maximum gray value is less than 256 then each pixel is represented by 1-byte. Otherwise, it uses 2-bytes.
Attached below is an image on how to use the Read from Binary File.vi to read the image information as a 2D array of integer values. Alternatively, this 2D array of integers could be converted to an image data-type using the IMAQ ArrayToImage.vi.