imagemodel is used to reduce the number of variables in a problem by eliminating explicit equality constraints.
Consider the following equality constrained SDP.
A1 = randn(2,2);A1 = A1*A1';
A2 = randn(2,2);A2 = A2*A2';
A3 = randn(2,2);A3 = A3*A3';
z = sdpvar(3,1);
obj = sum(z)
F = [C+A1*z(1)+A2*z(2)+A3*z(3) >= 0, z(1)+z(2)== 1];
A model without the explicit equality constraint is easily obtained.
We solve the reduced problem, and recover the original variables
Note that this reduction is done automatically if you call solvesdp and use a solver that cannot handle equality constraints. Hence, there is typically no reason to use this command, unless some further manipulations are going to be performed.
Note that there is a function in the image processing toolbox that may infer with YALMIPs imagemodel command. To resolve this, simply remove the path to the image processing toolbox.