I haven't actually started working with LotRO's interface at all, so take what I say here with a grain of salt.
Most games for which I've done any interface art/run a site and looked at others' work has required that:
a) your .tga file be a
power of two (not multiple, power). 2, 4, 8, 16, 32, 64, 128, 256, 512. The image within the file can be whatever size, but the file itself must be power of two;
b) you also need to remember to save your alpha channel (transparency), so it has to be saved as a 32 bit not a 24 bit. RGB+alpha.
As I said, I don't know that for sure for LotRO, but it has applied for every other game I've seen so far.
What you've got so far looks good.
