@molly
It's a good little landscape image you have there, you have a good sping-feel to it with the birds and the overall elements. Moko is correct though that there is no real focus to the image, all elements are roughly equal of importance. When composing an image you want to avoid this - you want to "guide" the viewer into your image.
What I think might be an useful test here is to determine what you want the image to be "about". Is it about the birds sitting on their branch? Is it about a lonely tree stump in the forest? Decide which part of your image you want the viewer to look at first, then try to bring this element forwards. You can do this in various ways, the easiest is to use light. You could subtle dim parts of the image to bring focus to the element you want to bring focus to. This is not the only way to do it though, you could for example use colors or stronger outlines to bring forth the element you want.
As an example, take a look at the RAP image I did here:
http://griatch-art.deviantart.com/art/F ... -530153302, notice how the background is shaded gray and turning whiter towards her finger? This is a way to help tell the audience just where they should focus. The idea is that once they have "landed" in the pointing finger, they can look around at the rest of the image and take it in.
As for shadows, you are right that there is no clear light direction on an overcast day. But this doesn't mean you cannot use shadows if you want - even on a day without a visible sun you will still have a general light direction - the light comes from the sky above! In fact, on a gray day the colors you see in the world will best match their "real" colors (that is, a red ball will look its "real" red on a gray day whereas it will be affected by scattered light from the environment when you have more interesting light hitting it).
Anyway, I'm glad you are finding good use for MyPaint.

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Griatch