One way might use vertex, another way would just use mask() to cut the ellipse in the image.

Indeed. There are several mask() examples on the forum I believe.

Here is an example of the vertex route. Just remember that texture coordinates are normalized between 0-1.

Code Example

1. PImage img = loadImage("http://csmres.co.uk/cs.public.upd/article-images/clouds-88571.jpg");
2. int numParts = 80;
3. float diam = 100;
4.  
5. void setup() {
6.   size(400, 400, P2D);
7.   smooth();
8. }
9.  
10. void draw() {
11.   background(0);
12.   translate(mouseX, mouseY);
13.   beginShape();
14.   texture(img);
15.   float theta = TWO_PI / numParts;
16.   for (int i=0; i<numParts; i++) {
17.     float angle = theta * i;
18.     float x = cos(angle);
19.     float y = sin(angle);
20.     vertex(x * diam, y * diam, (x+1)/2, (y+1)/2);
21.   }
22.   endShape();
23. }