The character design is how the character is position on the page, the pose, how the white space is handled, the colors used, the proportions, the line quality, the form, etc. All of which are well done on that drawing. The colors form a nice unison, the white space is balanced, the proportions have a very pleasing form to them. for more information about design google "the principles of design"
Save images for the web in png almost always. It is only scaleable infinitely if its vector and saved in a vector format. SVG files can scale infinitely
I wouldn't pay more or less if you used vectors or vexels. All that matters is the finish product.