Portable Pix Map, an image format of the PNM family.  It is a part of
[PBM Plus] - a large numbers of image converters and manipulators available
on the internet.

Every pixel is represented by three bytes for red, green, and blue channel. No compression, no transparency, as far as [RS] is aware.

[Tk] images can be read and written in PPM format using [photo] [[as of which version?]].  See also [PGM].

[RS] 2006-06-02: Here is a reader that parses a "P3" format PPM string (like read from a file) and creates a [photo] from it:

 proc ppm-photo ppm {
   foreach {type w h max} $ppm break
   foreach {r g b} [lrange $ppm 4 end] {
      set r [expr {int(255.*$r/$max)}]
      set g [expr {int(255.*$g/$max)}]
      set b [expr {int(255.*$b/$max)}]
      lappend row [format #%02X%02X%02X $r $g $b]
      if {[llength $row] == $w} {
         lappend rows $row
         set row {}
      }
   }
   set im [image create photo]
   $im put $rows
   set im
 }
#-- Testing:
 set data {P3
 4 4
 15
 0 0 0 0 0 0 0 0 0 15 0 15
 0 0 0 0 15 7 0 0 0 0 0 0
 0 0 0 0 0 0 0 15 7 0 0 0
 15 0 15 0 0 0 0 0 0 0 0 0
 }
 ppm-photo $data
#-- produces the following image (zoomed by a factor of 9):
[http://mini.net/files/ppm.jpg]

----
[Category Graphics]