Haven't had this problem before, but what's probably relevant here is the Processing version and renderer.

Note that TGA will probably be larger than JPG, because the former is lossless and the latter is compressed.

I tested but the problem occurs with both 1.5.1 & 2.0b7.

You could try PNG instead of TGA, which is another lossless format.

But as said, lossless formats will be bigger and thus require more disk space.

Yes I will. Tried tif as well and works fine too.