Quite amusing, I did manage to fix those floating pieces of snow, but solution was quite different that one would first thought.
I did select transparent background with magic wand tool, then I inverted the selection, copied selection to clipboard, pasted it into PowerPoint, Scaled image down to 63% in PowerPoint and made a copy of it, then copied again to clipboard and pasted as new image to gimp, no more problem with floating snow!
Then I added new layer, moved it to behind, applied bucket fill to that layer RGB 232, 232, 232, but this is no good yet, even as I use chroma and pick correct color there is box around the tree, so in GIMP I did select only alpha channel of layer which tree image was on, then right clicked alpha channel and selected selection from channel, so I had tree selected perfectly, then I hide layer tree was on, activated layer of chroma color and inverted selection, after that with eraser tool using options of hard edge and large brush I did removed chroma color so that there was only shape of tree left, after that did unhide all layers and merged all layers, exported to dxt5 format and it actually works now with good shadows and all.
In Xpacker I chose shadows to be low and texture, now with max shadows setting there is no errors or issues that I can see of.
Well, except photo used for texture could be better, texture is really huge for a tree, also center of two trees are not properly set, that is of course because tree was not enough straight, but I think that could be fixed at image editor if user would be talented enough, I'm not. Oh and tree itself is quite huge too, I don't think that they will come much bigger than that, but with scaling you can easily make smaller versions naturally. At least lowest branches are what I think should be removed and put some kind of trunk there, or just removed.
You can horrify yourselfs with this, oh and icon is with black background because I was too lazy to change, but I think white would not be too good with snowy tree:
http://jtbo.pp.fi/tiedostot/rfactor/FinnishWinter.zip
So now I can actually try and make some trees for this track and I did wrote it up here so when I forgot, I can come and check how to do it, maybe useful for someone else too
edit: I just realized that those black lines at top and black outlines at tree does appear only when I tick box 'receive light', shadows seem not to have anything to do with that, but of course trees should receive light, again problem and I think it might still is texture to blame.