Q:

Create an image showing max intensities in a set of images?

I would like to use a set of two images with the same color scheme (img1 and img2) to produce a new image (img3) in which each pixel corresponds to the "higher pixel color value" of img1 and img2.
Imagine objects (e.g. disks) in img1 have the color col1:
col1= ColorData["TemperatureMap"][0.5]

In img2 the disks are at different positions (partly overlapping with img1), they should have the color col2:
col2= ColorData["TemperatureMap"][0.2]

For the resulting image img3 we should check if objects in img1 and img2 overlap there. If so, then the pixel gets the color col1 (because 0.5 > 0.2).
If at a pixel only one of the images has an object, then color remains the same. A scheme with only three colors might look like this:

blue AND red --> red
blue AND background --> blue
background AND red --> red

A:

Nice Problem.  
Perhaps something like:
f[x_] := List @@ (ColorData["TemperatureMap"][x]);
s = Table[(f@x) -> x, {x, 0, 1, .001}];
k = Nearest@s;

Usage:
c = DensityPlot[#, {x, 0, 1}, {y, 0, 1}, ColorFunction -> "TemperatureMap", Frame -> None, 
               PlotRangePadding -> 0, PlotPoints -> 100] & /@ {x, y};
m = Map[k, (List @@@ ImageData@#), {2}] & /@ c;
GraphicsRow[Join[c, {Image@Map[f, MapThread[Max, m, 2], {2}]}]]

Although I think there is still a small problem in the interpolation in the middle of the max zones.

A:

Yes, you can use ImageApply. For example, if img1 and img2 are two images of the same size, then 
ImageApply[Max, {img1, img2}]

takes the max of each channel.

