Hi @aksinya,
I think both digital and paintings are same the only difference is that one use traditional art supply to create it and another one use digital artsupply to create that. So as far as category is concern, you can add both of them.
But each category has its own limitation. Painting can include TEXTURE automatically, on the other hand it has limited pallet and vibrancy, depends on art supply. On the other hand digital has many capabilities that convention process can't provide you, but you need to formulate artificial texture in your art. Every thing is possible in either way, but if you compare them, digital should stand out, if it is made by equally talented artist. So as far as category is concern, you can merge th to show others. Bit you can't compare them.
Hope It might be helpful.

Have a Nice Day ...