tint works like most other similar commands in Processing, it's basically an "on/off" switch. If you've set it to a specific tint, f.ex.

tint( 255, 0, 0 ); // Red tint

you need to reset it

tint( 255, 255 );  // No tint

for the next item that will react to it (in this case its the next PImage). Think of how you you draw two rectangles, one yellow and one red. Before drawing each rectangle you'd need to set the fill for the rectangle.

1. PImage img1, img2;
2. void setup(){
3.   size( 400, 400 );
4.   img1 = loadImage( "Jellyfish.jpg" );
5.   img2 = loadImage( "Lighthouse.jpg" );
6. }
7. void draw(){
8.   background( 255, 0, 0 );
9.   tint( 255, 255 );
10.   image( img1, 0, 0, 300, 300 );
11.   tint( 255, 100 );
12.   image( img2, 100, 100, 300, 300 );
13. }