Have the supply ddepots have the image of a small tiberium field. Then have a supply truck (harvester) come and take some tiberium.
However, this has 2 bugs that I can think of:
1). Possibly less types of Tiberium?
2). Tiberium doesn't regrow.

Here's my idea for the oil derricks:
Give it an image of a mine with Tiberium around it. Then you get free tiberium...
Any comments?