DXTBmp is for the editing of Game textures.
These have to be a power of 2 in width and height so it will automatically resize any that aren`t. This has always been the way it works and there shouldn`t be any other circumstances where it would perform resizing.
Are the images you are using a non-valid size?
Valid power of 2 sizes are 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 and 2048. Anything else will be changed to the nearest valid size

If the imaes are valid sizes the DXTBmp shouldn`t change them. I can`t think of any reason this would happen.
I certainly haven`t changed anything in the code for quite a while