With Docky I have really good icons except for the jDownloader one. I installed it from the official repo. How can I change the icon Docky uses?
Probably there are scalable vector graphics (*.svg files) for all your applications except of jdownloader (which has only a small png file or so).
There are several folders in
/usr/share with icons and images. One is
/usr/share/pixmaps/. Probably in one of the two you'll find a icon
jdownloader.png. If you don't find it, search for the *.desktop file (the launcher) and open it with your favourite text editor. You'll find a line "Icon=XY" and XY is the name of the icon (without the file ending). You can then search
/usr/share for this name. The desktop file is located in
/usr/share/applications but you get it even faster by right-click in your application menu and then "Add this launcher to desktop".
If you found the icon file used, you can replace it with another one with higher resolution. Or even better if you have a SVG version of it, just place the svg file there (no need to remove the other file then). But unfortunately it doesn't seem as if there is a SVG file for the official logo available. But I found this one.