for the smaller things - a scanner is your best choice. there is a guideline of how to shoot things floating in here, it might be under help. abbie has the list. i don't shoot them myself but if i did i would hang it flat on a wall - no frame, i guess lean it then. tripod, a tent made of sheet over it to diffuse light. add lights on both sides and try that. or experiment with a flash shooting it from the side to highlight detail if there is any. but diffusing that without hot spots is always a pain. which is why the scanner is best.
the tree either needs a better camera, or for you to zoom into it, take many images, and stitch them.
glare will never make an image sell, and anyone who buys things - they don't want glare built into the piece. it's not that it won't sell - they won't print it. they will make you do it again or you'll lose the sale. you have like a 2 day window to fix it if you get the dreaded letter.
for a painting they should look like the original. they shouldn't look dark, distorted, have glare, shouldn't look crooked, can't have crop marks, need to be sharp, can't be noisy or blocky.
this won't print it's blurry with motion blur it's also dark on the edges
it's hard to say if this will print or not, the left looks like glare to me, and it will probably be flagged, yet it could be sunlight. it doesn't look sharp, but up close i can see faint canvas. either way i think you'll be challenged on it.
this should print, though your name is a touch close to the edge (if your name is cut off they won't print it, but i think this is right on the line of being ok).
---Mike Savad