You can load from other file formats, but you lose any advantages of the different texture formats that OpenGL ES allows you - i.e. everything will be 32bpp RGBA8888 or 24bpp RGB888 which is far from optimal for most situations. Compared this to 2bpp or 4bpp PVRTC, for instance.
The procedure would be:
- open file
- read/decompress data -> RGBA8888
- pass to GL
The only advantage to be gained is in storage size (at least for png and jpg) and these formats can come directly from other sources than PVRTexTool.
All the formats you mention can be converted to .PVR via PVRTexTool and with a little scripting this can be done automatically - see the SDK examples.
Do you have a particular reason that you want to load directly from these formats?
on page 6 in the PVRTexTool.Reference Manual.1.11f.External.pdf
An image file of the format (BMP, TGA, GIF, PCX, JPG and PNG). MIP-map information will be discarded when these formats are chosen – use Save MIP-level from the Edit menu for this functionality. A decompressed version of the encoded data that will be saved.
According to the above step,I would save original bmp as new bmp file.