Krita
Krita is what I use for all of my raster drawings. Example: Hoo?, specifically Hoo? The Tarot Deck!
(For vector images, I use Inkscape. I also have a page about software I use.)
Reference images
The reference image tool is a fantastic idea. It’s currently a little awkward to use, but I’m sure the interface will improve with time.
To do stuff with reference images, select the tool icon, which looks like a pushpin (thumbtack). Options such as decreasing the opacity of the reference images are then available under the "Tool Options" panel.
To add an image from the clipboard as a reference image:
Edit > Paste as Reference Image (Ctrl + Shift + R)
Sometimes I want to show and hide references quickly.
Doing this with the tool options is really slow.
Thankfully, Krita uses a standard "File" menu with shortcuts,
so the sequence to show/hide all reference quickly is
currently: Alt-v h h Enter (Alt-v followed by h, h, and enter.)
(By the way, this is why the standard "File" menu remains peak accessibility and discoverability for my money and I hope applications like Krita keep it for as long as it is still allowed by the UX Powers That Be.)