1. A method for providing an on-demand software execution service comprising the following steps: receiving, from at least one terminal, at least one request for executing at least one software package, by a virtual machine launched on a virtual machine launching server, from files of a set of image files for launching virtual machines, the set of image files comprising: a first sub-set including at least one first image file representing a first state of a virtual machine wherein an operating system is installed on this machine; a second sub-set including at least one second image file representing a set of operations for bringing the virtual machine from the first state, in which said operating system is installed on the machine, to a second state wherein a software package is installed on the machine; a third sub-set including at least one third image file representing a set of operations for bringing the virtual machine from the second state, in which said operating system and the software package are installed on the machine, to a third state wherein said software package is launched on the machine; and a fourth sub-set including at least a context image file, which links virtual elements simulated by the virtual machine to actual physical elements of the terminal corresponding to said virtual elements, selecting at least four image files from among each of the first, second, third, and fourth sub-sets in order to execute said at least one software package by launching a virtual machine, three image files among the at least four image files corresponding to said at least one software package and to an operating system associated with said at least one software package; selecting, from a set of servers for launching virtual machines, at least one server to which the request will be directed, according to a rule for distributing the execution load of virtual machines by the servers of said set of servers, one or more image files among the set of image files being shared between the set of servers; concatenating the content of the at least four image files; and executing a virtual machine in the selected server based on this concatenation.