@retroen: you're welcome ^^

@kiokurashi: yes, what I meant is that versions of the piece that are scaled-down and non-pixel art (e.g. with hundreds of colors generated by a possible automatic smoothing feature of the resize tool used) are generally not accepted as a preview.
(Also, in the meanwhile I checked the forum and found this, which explains the whole preview-related part).

Anyway,  I'll update my first post on this piece, since all issues are fixed by now ^^

Isn't that the point of a preview?

i think its fine to have a small resize of the main piece for your preview :)

Thanks for info. I hope I've attached correct files now :)

It looks like you uploaded a scaled-down version of both this and choo choo (I saw the originals on your deviantart page, though, and they look cool).

Btw, fyi: I'm not 100% sure about this, but I think that the preview  files (aka Small files) shouldn't be a scaled-down version of the piece you submit.

(Edit: I also replaced "resize" with "scale down" for sake of clarity)