Standard behaviur if GL_ARB_texture_non_power_of_two extension is not supported is to rescale image to power of 2 dimensions.

DisableNPOTSupportCheck: Boolean = False;

ImagingOpenGL (at line 182)

NPOT extension is exposed only when HW has full support for NPOT textures but some cards (ATI Radeons, some other maybe) have partial NPOT support. Namely Radeons can use NPOT textures but not mipmapped. If you know what you are doing you can disable NPOT support check so the image won't be rescaled to POT by seting DisableNPOTSupportCheck to True.