I'd install krita
https://krita.org/en/
while it's installing and launching
download the palette from
https://lospec.com/palette-list/sunset as GIMP GPL
then
you right click on the image>open in new tab
right click again>copy image
In krita you click on New Image>Create from clipboard (you should now see the tree in the preview)
then you import the palette: settings>manage resources> import resource> select the GIMP GPL file you downloaded
Load the palette: add the palette docker (Settings>Dockers>Palette), load the palette (look at the gif)
(you need to click on the multicolored square)
then you draw your decoration on the tree, using colors from the palette:)
to save it: file>export
Upload it on a image hosting website (I use
Imgur ), copy the image address and upload on the forums