Ah right, that's strange. How are you downsampling the image? Assuming the white shows up in the 512 texture and isn't something related to your engine.
Zoom like hell on your texture Photoshop got a REALLY bad habit with the image resize, it OFTEN let 1px empty on a side, sometime it will even resize it to 511px or 513 px even when you wrote 512
If you try and scale down a layer or a document, that hasn't been flattened, using the "Bicubic sharper" -algorithm, then you WILL get alpha values on the border pixels (1px border all around).
The same thing applies when you select "Bicubic: automatic" and scale something down.
This is something you need to keep in mind or you might end up with some ugly seams on your texture. To get around it, make a duplicate of your document, flatten it and then scale down. If you want to scale down prior to flattening the doc, then you can use the "Bilinear" -algorithm instead.
Replies
http://wiki.polycount.com/EdgePadding
The same thing applies when you select "Bicubic: automatic" and scale something down.
This is something you need to keep in mind or you might end up with some ugly seams on your texture. To get around it, make a duplicate of your document, flatten it and then scale down. If you want to scale down prior to flattening the doc, then you can use the "Bilinear" -algorithm instead.
Try this: Convert the image image to 32 bit, resize, convert back to 8 bit. Does that fix it?