you will have to use something like Paintshop, Irfanview,Photoshop or other program and resize the files in multiples of 2 pixels , as in 2x2,4x4,......64x64,128x128 or 64x128 etc. and replace them.
The other comments in the thread are all correct. I would note one other thing, even though your post doesn't suggest that it is your problem. Although the ~.tga format supports some form or forms of compression, when I accidentally attempted to use a compressed ~.tga file in TRS, it was not a successful exercise, but worked fine when I used the same file in an uncompressed format.