Pretty good considering you don't have a tablet.
Firstly I think you need to work on your proportions and getting the face to become accurate, anything else is secondary imo. Many artists will say drawing is the most important part of painting, if the drawing's not accurate then the painting won't help.
K, so it's not bad - you may even want to leave it how it is.
I will offer some tips anyway, you can take 'em or leave 'em or even come back to them one day =)
I am learning myself so I won't give you a thesis on drawing the face lol.
no1. Flip canvas - I am sure artrage has this feature. When you are drawing a face, particularly in the initial block in, you want to flip the canvas regularly, horizontally and vertically - this will show any errors in proportions, like a wonky nose, uneven lips etc etc. A face is all about symmetry, this will show you your eyes are a good width apart, if they're the same height etc.
no2. Proportions - What I have found helped a huge amount was Loomis' book - "drawing the head and hands" http://acid.noobgrinder.com/Loomis/Andrew%20Loomis%20-%20Drawing%20the%20Head%20and%20Hands.rar
There are some excellent exercises in this book, and they don't take long to get the hang of - they will improve your face mark making a lot. I spent a day on the book and immediately I could draw a better head without any reference material.
no3. Line weight -Instead of drawing uniform lines and thickness all over; start with faint lines - then use heavier lines where you want to viewer to look. E.g. define the eyes - not everything has to be of equal importance in a portrait.
no4. As I said it's not too bad right now, the left eye is higher than the right eye though; flipping the canvas will display this issue to you . Same with the chin/lower jaw and the nose, one side is slightly lower than the other.
Well you seem to have a good grasp of light source, and where to shade. I guess the rest is practice!
One more thing, just drawing a head and neck doesn't make a good portrait. at least put as much as shoulders in =) shouldn't be too hard!
Anyway, don't feel you have to change anything, I just like to give a critique that I would expect someone to give to me, whether I use it or not is another thing. But I think it's just nice to be made aware of things :-)
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