Step 1: Logically, We Close the Can Firmly.
Step 2: How Do We Eliminate the Oxygen?
In the upper left corner of the photo you can see a dropper with lighter fuel. Four or five drops are sufficient. More than that can alter the painting chemically.
Step 3: Warning: Adults Only!
Step 4: Put the Lid
The fire fades quickly because the oxygen wastes away, and then the painting will last very much more time without losing it's quality significantly.
If there is little painting and the can is of big size (1 liter or more), it is probable that the can is deformed due to the interior vacuum. But that doesn't affect in anything to the painting, and there is not any possibility that the can can produce spill of painting due to that contraction.