Hey I had to do this once too I did a small tile engine and such but now I''m trying to experiment on BSP''s and see how that works for settings like castles in rpgs and caves and such but I might go back and work on my tile engine again since I liked it heres my modification off Nehe''s .tga loader basically its the same thing though

  TextureImage::Extract(int x, int y, int ex_width, int ex_height){	unsigned char *src = NULL,*dest = NULL, *dest2;	unsigned int type = GL_RGBA;	int bytes_per_pixel = bpp/8;	unsigned int id;	dest = new unsigned char[ex_width * ex_height * bytes_per_pixel];	memset(dest,0,sizeof(dest));	x = x * (ex_width);	y = y * (ex_height);		src = buffer + (y * (width * bytes_per_pixel) + (x * bytes_per_pixel));	dest2 = dest;	for(int index = 0; index < ex_height; index++)	{		memcpy(dest2, src, (ex_width * bytes_per_pixel));		dest2 += (ex_width * bytes_per_pixel); 		src  += (width * bytes_per_pixel);	}	glGenTextures(1, &id);	glBindTexture(GL_TEXTURE_2D, id);	if(bpp == 24)		type = GL_RGB;	glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);   	gluBuild2DMipmaps(GL_TEXTURE_2D, type, ex_width, ex_height, type, GL_UNSIGNED_BYTE,dest);	free(dest);	dest2=NULL;	src = NULL;	return(id);}  

you can mess around with that as long as you have the buffer saved that you read in it works so like in your class keep like a unsigned char *buffer; where all the image data is kept only thing that is weird on this that I havent figured out really why it is is that lets say you have a 2 rows of cells and 3 colums
1 2 3
4 5 6
the cell 4 is located at 0,0 so you would call the function like
Extract(0,0,32,32) //if the tiles are 32x32 and that will give you the image of cell 4 if you want cell 1 you call Extract(0,1,32,32) yeah its weird like taht but I havent figured out why it does it...if anyone knows please tell