Ahh!! Kondratbl. You don't have to make the main image 100x100. You just need the preview to be 100x100, and you should only show a portion of the image rather than shrink it all down (or else you'll end up with a messy image like this) 

So you can submit the main image as large as you want!

I'm just not sure how to cramp an originally larger image into 100x100 pixels without getting this result (the original is in .png as well).

2428 colors. This image may have been saved as a jpeg first and then saved as a png. I see a lot of colors bleeding into each other and blooming, (what you'd normally see from a .jpeg)

Great job!  I like the gloomy feel you gave this piece.   The textures look great on the snow.  It does look like you've got something wrong with the compression of this submission, though. Even though it doesn't look like it initially, the image specs HERE showed that you have over 2000 colors.  That would definitely need to be rectified before it could be accepted into the gallery, but checking the compression settings on whatever program you're using could prove it to be a simple fix.  Great job, and keep working on these great concepts!