Pyglet program to draw a single textured square - same as texturedSquare.py, except that it uses PIL (Python Image Library) to load the texture image, and then explicitly makes the OpenGL calls to create the texture (so that different tex parameters can be used).

texturedSquare_PIL.py

import sys, time, math, os, random
import Image
from pyglet.gl import *

window = pyglet.window.Window()
keyboard = pyglet.window.key.KeyStateHandler()
window.push_handlers(keyboard)


def loadTexture(filename):
    img = Image.open(filename).transpose(Image.FLIP_TOP_BOTTOM)
    textureIDs = (pyglet.gl.GLuint * 1) ()
    glGenTextures(1,textureIDs)
    textureID = textureIDs[0]
    print 'generating texture', textureID, 'from', filename
    glBindTexture(GL_TEXTURE_2D, textureID)
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, img.size[0], img.size[1],
                 0, GL_RGB, GL_UNSIGNED_BYTE, img.tostring())
    glBindTexture(GL_TEXTURE_2D, 0)
    return textureID


class TexturedSquare:
    def __init__(self, width, height, xpos, ypos, texturefile):
        self.xpos = xpos
        self.ypos = ypos
        self.angle = 0
        self.size = 1
        self.texture = loadTexture(texturefile)
        x = width/2.0
        y = height/2.0
        self.vlist = pyglet.graphics.vertex_list(4, ('v2f', [-x,-y, x,-y, -x,y, x,y]), ('t2f', [0,0, 1,0, 0,1, 1,1]))
    def draw(self):
        glPushMatrix()
        glTranslatef(self.xpos, self.ypos, 0)
        glRotatef(self.angle, 0, 0, 1)
        glScalef(self.size, self.size, self.size)
        glColor3f(1,1,1)
        glEnable(GL_TEXTURE_2D)
        glBindTexture(GL_TEXTURE_2D, self.texture)
        self.vlist.draw(GL_TRIANGLE_STRIP)
        glDisable(GL_TEXTURE_2D)
        glPopMatrix()


@window.event
def on_draw():
    glClearColor(0, 0.3, 0.5, 0)
    glClear(GL_COLOR_BUFFER_BIT)
    square1.draw()
    

def update(dummy):
    global square1
    if keyboard[pyglet.window.key.A]:
        square1.xpos -= 5
    if keyboard[pyglet.window.key.D]:
        square1.xpos += 5
    if keyboard[pyglet.window.key.W]:
        square1.ypos += 5
    if keyboard[pyglet.window.key.S]:
        square1.ypos -= 5
    if keyboard[pyglet.window.key.UP]:
        square1.size *= 1.1
    if keyboard[pyglet.window.key.DOWN]:
        square1.size /= 1.1
    if keyboard[pyglet.window.key.LEFT]:
        square1.angle += 5
    if keyboard[pyglet.window.key.RIGHT]:
        square1.angle -= 5


square1 = TexturedSquare(120, 120, 300, 200, "texture.jpg")

pyglet.clock.schedule_interval(update,1/60.0)
pyglet.app.run()

hey I know this post is ancient but its a top search result on duckduckgo so I may as well mention this
image.tostring() is deprecated, use image.tobytes() instead, this will otherwise cause an access violation
if the loaded texture has an alpha channel, the resulting texture will be corrupted,
to fix add if img.mode == "RGB": img.putalpha(255) right after loading the image and change GL_RGB to GL_RGBA
also fixed the print function for newer python versions

(copy of your load function with changes made)

def loadTexture(filename):
	img = Image.open(filename).transpose(Image.FLIP_TOP_BOTTOM)
	if img.mode == "RGB":
		img.putalpha(255)
	textureIDs = (pyglet.gl.GLuint * 1)()
	glGenTextures(1, textureIDs)
	textureID = textureIDs[0]
	print('generating texture', textureID, 'from', filename)
	glBindTexture(GL_TEXTURE_2D, textureID)
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR)
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR)
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, img.size[0], img.size[1],
	             0, GL_RGBA, GL_UNSIGNED_BYTE, img.tobytes())
	glBindTexture(GL_TEXTURE_2D, 0)
	return textureID