Don't make the chart 3D as this is misleading. Keep it 2D.
Consider leaving text labels off the image to increase its use in multiple wikis (for example, put sections in different colours, and then in a caption/on the image description page, state what the colours stand for). Numbers are fairly neutral. See Template:Legend
Consider if a pie chart is the best representation for your data. Pie charts are best when there is a fixed, limited amount of "stuff" to go around, such as votes (limited by the population at a specific time) or budgets. Populations can grow - would the data be more flexible if presented as a bar chart, which can grow?
Graphs are best as SVGs, which can be easily edited by other users. PNG and GIFs are also OK but not as flexible. They should never be JPGs.
