Your best bet is to use InkScape and convert your graphics to SVG. Polygons can be filled with rasters, but points and markers need SVG. If your symbol is available in a font, you can also use Font markers.
It's fairly straightforward:-
- Load bitmap (png, jpg etc) into Inkscape with File > Open
- File > Save as (use
Plain SVG rather than default
- You can now bring your SVG in as a symbol.
If it doesn't show up in QGIS, you may need to add your directory to the list QGIS uses to find SVG symbols. You can find this under Settings > Options > System > SVG Paths
Sometimes your symbol might show a background colour, when you want transparency. If that happens you can use GIMP on your original bitmap.
To "knock out" the background in the GIMP
- create a copy of original with File > Export As
- on the copy use Gimp's Color > Color to Alpha
- save the new copy with File > Export As
- repeat steps in inkscape