When specifying image data in OpenGL, how does endian ordering affect the pixel packing? Does using GL_BYTE vs. a pre packed format like GL_UNSIGNED_INT_8_8_8_8 change the way the pixels are unpacked?

What's the easiest way to make texture loading cross platform? I was planning on using SDL_image, but is there something better? If loading is slow, that's not such a big deal, but uploading to texture memory needs to be fast. There will be a lot of dynamic uploading.

yes, RGBA/UNSIGNED_BYTE is treated quite differently from RGBA/UNSIGNED_INT_8_8_8_8. The former is independent of endianness, the latter is not.