Xournal displays PNGs ~/<<colour>>.png as icons

Ubuntu 10.04.4 LTS
xournal: installed: 0.4.5-2

I opened xournal and it was completely unusable. The toolbar was about 500 pixel high. The icon for selecting black colour was monstrous. I was looking at different things, tried a reinstall but nothing helped. Then I remembered that I created a black bitmap file in my home directory a while ago, and out of curiosity I renamed it - and the issue was gone. I didn't recognize my own graphic file since it was actually a pure black graphic.

So: when placing a PNG named black.png or white.png or <<colour>>.png in the home directory it will be used by xournal as an icon
for selecting the corresponding colour. That's a really unexpected behavior.